Light-Emitting Element, Light-Emitting Device, Electronic Device, Lighting Device, and Lighting System

ABSTRACT

A light-emitting element that contains a fluorescent compound, which has high efficiency is provided. A light-emitting element in which the proportion of delayed fluorescence to the total light emitted from the light-emitting element is higher than that in a conventional light-emitting element is provided. Emission efficiency of the light-emitting element containing a fluorescent compound can be improved by increasing the probability of TTA caused by an organic compound in an EL layer, converting energy of triplet excitons, which does not contribute to light emission, into energy of singlet excitons, and making the fluorescent compound emit light by energy transfer of the singlet excitons.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to a light-emittingelement, a light-emitting device, an electronic device, and a lightingdevice. In addition, one embodiment of the present invention relates toa lighting system. Note that one embodiment of the present invention isnot limited to the above technical field. The technical field of oneembodiment of the invention disclosed in this specification and the likerelates to an object, a method, or a manufacturing method. In addition,one embodiment of the present invention relates to a process, a machine,manufacture, or a composition of matter. Specifically, examples of thetechnical field of one embodiment of the present invention disclosed inthis specification include a semiconductor device, a display device, aliquid crystal display device, a light-emitting device, a power storagedevice, a storage device, a method for driving any of them, and a methodfor manufacturing any of them.

2. Description of the Related Art

In recent years, research and development of light-emitting elementsusing electroluminescence (EL) have been actively conducted. In a basicstructure of such a light-emitting element, a layer containing alight-emitting substance (an EL layer) is interposed between a pair ofelectrodes. By applying a voltage between the pair of electrodes of thiselement, light emission from the light-emitting substance can beobtained.

Since the above light-emitting element is a self-luminous type, adisplay device using this light-emitting element has advantages such ashigh visibility, no necessity of a backlight, and low power consumption.Further, such a light-emitting element also has advantages in that theelement can be formed to be thin and lightweight, and that response timeis high.

It is said that the light emission mechanism of a light-emitting elementis as follows: when a voltage is applied between a pair of electrodeswith an EL layer including a light-emitting substance providedtherebetween, electrons injected from a cathode and holes injected froman anode recombine in the light emission center of the EL layer to formmolecular excitons, and energy is released and light is emitted when themolecular excitons relax to the ground state.

The excited states of an organic compound in which molecular excitonsare formed include a singlet excited state (S*) and a triplet excitedstate (T*), and light emission from the singlet excited state isreferred to as fluorescence, and light emission from the triplet excitedstate is referred to as phosphorescence. The statistical generationratio thereof in the light-emitting element is considered to beS*:T*=1:3. In other words, a light-emitting element containing acompound emitting phosphorescence has higher emission efficiency than alight-emitting element containing a compound emitting fluorescence.Therefore, light-emitting elements containing phosphorescent compoundscapable of converting a triplet excited state into light emission hasbeen actively developed in recent years.

Among light-emitting elements containing phosphorescent compounds, inparticular, a light-emitting element that emits blue light has not yetbeen put into practical use because it is difficult to develop a stablecompound having a high triplet excited energy level. For this reason,the development of a light-emitting element containing a more stablefluorescent compound has been conducted and high efficiency of alight-emitting element containing a fluorescent compound (fluorescentlight-emitting element) has been required.

In the light-emitting element containing a fluorescent compound,triplet-triplet annihilation (TTA) is known as a light emissionmechanism capable of converting part of a triplet excited state intolight emission. The term TTA refers to a process in which, when twotriplet excitons approach each other, excited energy and spin angularmomentum are exchanged and transferred to form singlet excitons.

As a compound in which TTA occurs, a compound including an anthraceneskeleton is known. Non-Patent Document 1 discloses that the use of acompound including an anthracene skeleton as a host material achieveshigh external quantum efficiency in a light-emitting element that emitsblue light. It also discloses that the proportion of the delayedfluorescence due to TTA to the total light emitted from thelight-emitting element using a compound including an anthracene skeletonis approximately 10%.

PATENT DOCUMENT Non-Patent Document

-   [Non-Patent Document 1]-   Tsunenori Suzuki et al., Japanese Journal of Applied Physics, Vol.    53, 052102 (2014)

SUMMARY OF THE INVENTION

One embodiment of the present invention is to provide a light-emittingelement which contains a fluorescent compound and has high efficiency.Another embodiment of the present invention is to provide alight-emitting element in which the proportion of delayed fluorescenceto the total light emitted from the light-emitting element is higherthan that in a conventional light-emitting element. Another embodimentof the present invention is to provide a novel light-emitting element.Another embodiment of the present invention is to provide a novellight-emitting device. Another embodiment of the present invention is toprovide a novel light-emitting device, a novel electronic device, or anovel lighting device. Another embodiment of the present invention is toprovide a lighting system in which energy saving of a light-emittingelement that is a light source is achieved. Note that the description ofthe object does not disturb the existence of other objects. In oneembodiment of the present invention, there is no need to achieve all theobjects. Other objects will be apparent from and can be derived from thedescription of the specification, the drawings, the claims, and thelike.

According to one embodiment of the present invention, emissionefficiency of a light-emitting element containing a fluorescent compoundcan be improved by increasing the probability of TTA caused by anorganic compound in an EL layer, converting energy of triplet excitons,which does not contribute to light emission, into energy of singletexcitons, and making the fluorescent compound emit light by energytransfer of the singlet excitons.

As one of methods for efficiently converting the energy of tripletexcitons into the energy of singlet excitons, energy transfer andintersystem crossing which are caused by the Förster mechanism are used.The Förster mechanism is a mechanism in which energy is transferred byresonance, which occurs more easily when the following conditions aresatisfied: organic compounds (host materials) containing tripletexcitons approach each other with an intermolecular distance of 1 nm to10 nm, the oscillator strength for transition from the lowest tripletexcited level (referred as a T₁ level) of the organic compound (hostmaterial) to one of levels of triplet excited states which are higherthan the T₁ level of the organic compound (referred as T_(n) levels) ishigh (energy absorption in transition is high), and the like.

One embodiment of the present invention is to provide a light-emittingelement in which energy transfer by the Förster mechanism is likely tooccur so that the probability of TTA in an EL layer of a light-emittingelement is increased.

One embodiment of the present invention is a light-emitting elementincluding an EL layer between an anode and a cathode. The EL layerincludes a light-emitting layer. The light-emitting layer comprises afirst organic compound. A difference between the T₁ level of the firstorganic compound and one or more of T_(n), levels of the first organiccompound is less than the sum of the T₁ level and 0.6 eV. Note that thelevels of triplet excited states which are higher than the T₁ level arereferred to as T_(n) levels in this invention. That is, one or more ofthe T_(n) levels of the first organic compound preferably have an energylevel close to twice the T₁ level of the first organic compound, becauseenergy transfer from the T₁ level to the T_(n) levels is facilitated.

In the above structure, an energy difference between any one of theT_(n) levels of the first organic compound and any one of levels ofsinglet excited states of the first organic compound is 1 eV or less.Note that the levels of the singlet excited states of the first organiccompound includes an S₁ level (the lowest singlet excited level) andS_(n) levels (levels that are higher than the S₁ level). This case isalso preferable because intersystem crossing easily occurs as the energylevel of any one of the T_(n) levels of the first organic compound iscloser to that of any one of the S₁ level and the S_(n) levels of thefirst organic compound.

Thus, in the above structure, an energy difference between any one ofthe T_(n) levels of the first organic compound and any one of the S₁level and the S_(n) levels of the first organic compound is 1 eV orless.

In each of the above structures, an oscillator strength for excitationfrom the T₁ level of the first organic compound to one of the levels ofthe triplet excited states higher than the T₁ level of the first organiccompound is 0.0015 or more.

In each of the above structures, the first organic compound is anorganic compound (host material) in which TTA can be efficiently causedand triplet excitons, which do not contribute to light emission, areconverted into singlet excitons. Thus, the first organic compoundpreferably includes a tetracene skeleton or an anthracene skeleton.

In each of the above structures, the first organic compound hasStructural Formula (100) or Structural Formula (110).

The above-described first organic compound is an organic compound (hostmaterial) in which TTA can be efficiently caused, triplet excitons,which does not contribute to light emission, can be efficientlyconverted into singlet excitons, and the proportion of delayedfluorescence to the total light emitted from the organic compound ishigh because the triplet exciton has long excitation lifetime.

Thus, in the light-emitting element which includes the first organiccompound in the light-emitting layer and which has the above structure,the light-emitting element exhibits delayed fluorescence, and emissionintensity of the delayed fluorescence component to the emissionintensity of the total light emitted from the light-emitting element isat least 5% or more, preferably 10% or more, further preferably 15% ormore for obtaining higher efficiency. Note that in the case where theenergy of triplet excitons is converted into the energy of singletexcitons through TTA, the proportion of delayed fluorescence due to TTAto the total light emitted from the light-emitting element can beincreased because the proportion of singlet excitons having alight-emitting property is increased. Note that in the case where asecond organic compound that is a guest material is included in thelight-emitting layer, because light emission may occur due to directrecombination of carriers in the guest material without the hostmaterial, the proportion of delayed fluorescence to the total lightemission may be decreased compared to the case of an element structurewhich is the same as that of the light-emitting element except for notincluding the guest material.

In the above structures, the light-emitting layer comprises the firstorganic compound and the second organic compound, and an S₁ level of thefirst organic compound is higher than an S₁ level of the second organiccompound. Moreover, light emission from the light-emitting element isderived from the second organic compound.

In the above structure, the second organic compound includes a pyreneskeleton.

In each of the above structures, a T₁ level of the second organiccompound is higher than the T₁ level of the first organic compound.

In each of the above structures, the EL layer includes a hole-transportlayer and a light-emitting layer. The hole-transport layer in contactwith the light-emitting layer is located between the anode and thelight-emitting layer. The hole-transport layer comprises a third organiccompound, and a T₁ level of the third organic compound is higher thanthe T₁ level of the first organic compound.

In each of the above structures, the EL layer includes anelectron-transport layer and a light-emitting layer. Theelectron-transport layer in contact with the light-emitting layer islocated between the cathode and the light-emitting layer. Theelectron-transport layer comprises a fourth organic compound and a T₁level of the fourth organic compound is higher than the T₁ level of thefirst organic compound.

Another embodiment of the present invention is a light-emitting deviceincluding the light-emitting element having any one of the abovestructures and one of a transistor and a substrate.

Another embodiment of the present invention is an electronic deviceincluding the light-emitting device having any one of the abovestructures and any one of a microphone, a camera, an operation button,an external connection portion, and a speaker.

Another embodiment of the present invention is an electronic deviceincluding the light-emitting device having the above-described structureand one of a housing and a touch sensor function.

Another embodiment of the present invention is a lighting deviceincluding the light-emitting device having the above-describedstructure, and any one of a housing, a cover, and a support.

The present invention includes, in its scope, not only a light-emittingdevice including the light-emitting element but also a lighting deviceincluding the light-emitting device. The light-emitting device in thisspecification refers to an image display device and a light source(e.g., a lighting device). In addition, the light-emitting deviceincludes, in its category, all of a module in which a connector such asa flexible printed circuit (FPC) or a tape carrier package (TCP) isconnected to a light-emitting device, a module in which a printed wiringboard is provided on the tip of a TCP, and a module in which anintegrated circuit (IC) is directly mounted on a light-emitting elementby a chip on glass (COG) method.

A lighting system that is another embodiment of the present inventionhas a structure in which driving of a light-emitting element that is alight source is controlled in accordance with external informationobtained from a sensor. Note that as the light-emitting element in thelighting system, the above-described light-emitting element which is oneembodiment of the present invention is preferably used.

That is, one embodiment of the present invention is a lighting systemincluding a sensor, a control unit, and a light-emitting element.Information detected by the sensor is input to the control unit. Thecontrol unit is configured to drive the light-emitting elementelectrically connected to the control unit on the basis of theinformation. The light-emitting element includes an EL layer between ananode and a cathode. The EL layer includes a light-emitting layer. Thelight-emitting layer includes a first organic compound. A differencebetween a T₁ level of the first organic compound and one or more ofT_(n) levels of the first organic compound is less than the sum of theT₁ level and 0.6 eV.

Another embodiment of the present invention is a lighting systemincluding a sensor, a control unit, and a light-emitting element. Thecontrol unit includes a communication unit, a CPU, and a memory. Thememory includes a program for driving the light-emitting element basedon exterior information. The communication unit is configured to sendthe exterior information acquired by the sensor to the CPU. The CPU isconfigured to drive the light-emitting element by reading out theprogram from the memory and executing the program. The light-emittingelement includes an EL layer between an anode and a cathode. The ELlayer includes a light-emitting layer. The light-emitting layer includesa first organic compound. A difference between a T₁ level of the firstorganic compound and one or more of T_(n) levels of the first organiccompound is less than the sum of the T₁ level and 0.6 eV.

According to one embodiment of the present invention, a light-emittingelement containing a fluorescent compound, which has high efficiency canbe provided. According to one embodiment of the present invention, alight-emitting element in which the proportion of delayed fluorescenceto the total light emitted from the light-emitting element is higherthan that in a conventional light-emitting element can be provided.According to one embodiment of the present invention, a novellight-emitting element and a novel light-emitting device can beprovided. A novel light-emitting device, a novel electronic device, or anovel lighting device can be provided. A lighting system in which energysaving of a light-emitting element that is a light source is achievedcan be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1D each illustrate a mechanism of TTA;

FIG. 2 shows the directions of transition dipole moments and oscillatorstrengths in molecular structures;

FIG. 3 shows the directions of the transition dipole moments andoscillator strengths in molecular structures;

FIGS. 4A and 4B each illustrate a structure of a light-emitting element;

FIGS. 5A and 5B each illustrate a structure of a light-emitting element;

FIGS. 6A to 6C illustrate a light-emitting device;

FIGS. 7A and 7B illustrate a light-emitting device;

FIGS. 8A, 8B, 8C, 8D, 8D′-1 and 8D′-2 illustrate electronic devices;

FIGS. 9A to 9C illustrate an electronic device;

FIGS. 10A and 10B illustrate an automobile;

FIGS. 11A to 11D illustrate lighting devices;

FIG. 12 illustrates lighting devices;

FIGS. 13A and 13B illustrate an example of a touch panel;

FIGS. 14A and 14B illustrate an example of a touch panel;

FIGS. 15A and 15B illustrate an example of a touch panel;

FIGS. 16A and 16B are a block diagram and a timing chart of a touchsensor;

FIG. 17 is a circuit diagram of a touch sensor;

FIG. 18 illustrates a light-emitting element;

FIG. 19 shows attenuation curves;

FIG. 20 is a graph showing the current density vs. luminancecharacteristics of Light-emitting element 1 and Light-emitting element3;

FIG. 21 is a graph showing the voltage vs. luminance characteristics ofLight-emitting element 1 and Light-emitting element 3;

FIG. 22 is a graph showing the luminance vs. current efficiencycharacteristics of Light-emitting element 1 and Light-emitting element3;

FIG. 23 is a graph showing voltage-current characteristics ofLight-emitting element 1 and Light-emitting element 3;

FIG. 24 is a graph the current density vs. luminance characteristics ofLight-emitting element 2 and Light-emitting element 4;

FIG. 25 is a graph showing the voltage vs. luminance characteristics ofLight-emitting element 2 and the Light-emitting element 4;

FIG. 26 is a graph showing the luminance vs. current efficiencycharacteristics of Light-emitting element 2 and Light-emitting element4;

FIG. 27 is a graph showing voltage-current characteristics ofLight-emitting element 2 and Light-emitting element 4;

FIG. 28 shows emission spectra of Light-emitting element 1 andLight-emitting element 3;

FIG. 29 shows emission spectra of Light-emitting element 2 andLight-emitting element 4;

FIGS. 30A and 30B are ¹H NMR charts of an organic compound representedby Structural Formula (100);

FIGS. 31A and 31B shows an ultraviolet-visible absorption spectrum andan emission spectrum of the organic compound represented by StructuralFormula (100);

FIG. 32 shows results of LC/MS analysis of the organic compoundrepresented by Structural Formula (100);

FIGS. 33A and 33B are ¹H NMR charts of an organic compound representedby Structural Formula (110);

FIGS. 34A and 34B show ultraviolet-visible absorption spectra andemission spectra of the organic compound represented by StructuralFormula (110);

FIG. 35 shows results of LC/MS analysis of the organic compoundrepresented by Structural Formula (110);

FIGS. 36A, 36B1, and 36B2 show block diagrams of a display device;

FIG. 37 shows a circuit structure of a display device;

FIG. 38 is a cross-sectional structure of a display device;

FIGS. 39A and 39B illustrate a light-emitting element; and

FIG. 40 illustrates a structure of a lighting system.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the accompanying drawings. Note that the present inventionis not limited to the following description, and modes and detailsthereof can be variously changed without departing from the spirit andscope of the present invention. Therefore, the present invention shouldnot be construed as being limited to the description in the followingembodiments.

Note that the terms “film” and “layer” can be interchanged with eachother according to circumstances. For example, in some cases, the term“conductive film” can be used instead of the term “conductive layer,”and the term “insulating layer” can be used instead of the term“insulating film”.

Embodiment 1

In this embodiment, a mechanism of occurrence of TTA (triplet-tripletannihilation) in an EL layer of a light-emitting element is described.

There are various theories on the detail of the mechanism of TTA, and itis not defined clearly. In one embodiment of the present invention,energy transfer shown in schemes in FIGS. 1A to 1D is assumed to occurin TTA.

First, in the case where triplet excitons of two molecules of the samekind (a molecule A and a molecule B) exist adjacently as shown in FIG.1A, energy released when the molecule B transits from a T₁ level to anS₀ level is absorbed by the molecule A, and an electron at the T₁ levelof the molecule A transits to a T_(n) level. Thus, it is thought thatthe vicinity of an energy level that is higher than the T₁ level of themolecule A by the T₁ level of the molecule B is the T_(n) level to whichthe molecule A can transit.

Next, as shown in FIG. 1B, the electron transited to the T_(n) level ofthe molecule A undergoes intersystem crossing into an S_(n) level whoseenergy level is close to the T_(n) level with a certain probability.Furthermore, the electron at the S_(n) level transits to a S₁ level byinternal transition (FIG. 1C).

When the electron at the S₁ level transits to the S₀ level as shown inFIG. 1D, delayed fluorescence is generated.

As described above, using the excitation energy of one of two molecules,the other molecule transits to a higher level; thus, up to half of 75%of T₁ excitons can be extracted as emission. Since there is also 25% ofS₁ excitons generated by current excitation, the total internal quantumefficiency can be 62.5% at the maximum.

Note that in the case where intermolecular energy transfer through TTAdescribed in FIGS. 1A to 1D occurs, an increase in the efficiency ofenergy transfer from the molecule B to the molecule A in FIG. 1Aincrease the probability of transition from the T₁ level to the T_(n)level, thereby increasing the number of S₁ excitons that are finallygenerated. As mechanisms of such intermolecular energy transfer, twomechanisms, i.e., the Dexter mechanism (electron exchange interaction)and the Förster mechanism (dipole-dipole interaction) are given. Thefact that the energy transfer between triplet excitons (T₁-T₁) ispossible in either of the mechanisms is described below.

In the Dexter mechanism, electron spins of both two molecules are storedbefore and after energy transfer. Thus, when the electron spins of bothtwo molecules are stored before and after energy transfer, energytransfer by the Dexter mechanism is allowed. Note that energy transferthrough TTA has been mainly described using the Dexter mechanism.

Meanwhile, the rate constant k_(ET) of energy transfer in the Förstermechanism is expressed by Formula (1) below.

$\begin{matrix}{k_{ET} = {\frac{9000c^{4}\ln \; 10}{128\pi^{5}n^{4}N_{A}\tau_{0}^{a}}\frac{\kappa^{2}}{R^{6}}{\int{{f_{a}\left( \overset{\sim}{v} \right)}{ɛ_{b}(v)}\frac{\overset{\sim}{v}}{{\overset{\sim}{v}}^{4}}}}}} & (1)\end{matrix}$

c: The velocity of light, n: refractive index, N_(A): Avogadro number,τ₀: donor duration, R: intermolecular distance, K²: relative orientationfactor of transition dipole moments of A and B, {tilde over (v)}: wavenumber, f: light intensity per wave number standardized to Area 1, ε:absorption coefficient

Note that τ₀ in Formula (1) is the reciprocal number of radiation speedk_(r). Here, x is expressed as follows.

$k_{ET} = {\frac{9000c^{4}\ln \; 10}{128\pi^{5}n^{4}N_{A}}\frac{\kappa^{2}}{R^{6}}{\int{{f_{a}\left( \overset{\sim}{v} \right)}{ɛ_{b}(v)}\frac{\overset{\sim}{v}}{{\overset{\sim}{v}}^{4}}}}}$

Formula (1) can be represented by Formula (1′) using x.

k _(ET) =xk _(r)  (1′)

Furthermore, energy transfer efficiency φ_(ET) by the Förster mechanismis represented by Formula (2) below.

$\begin{matrix}{\varphi_{ET} = \frac{k_{ET}}{k_{r} + k_{nr} + k_{ET}}} & (2)\end{matrix}$

k_(r): radiative rate constant, k_(nr): non-radiative rate constant

Formula (2′) can be derived from Formula (2) and Formula (1′) asfollows.

$\begin{matrix}\begin{matrix}{\varphi_{ET} = \frac{{xk}_{r}}{k_{r} + k_{nr} + {xk}_{r}}} \\{= \frac{x}{\left( \frac{k_{r} + k_{nr}}{k_{r}} \right) + x}} \\{= \frac{x}{\left( \frac{1}{\varphi_{p}} \right) + x}}\end{matrix} & \left. \left( 2’ \right. \right)\end{matrix}$

φ_(p): phosphorescence quantum efficiency

In the case where the molecule A and the molecule B are anthracenederivatives, a radiation rate constant (k_(r)) of phosphorescentemission obtained from the anthracene derivatives is 1×10³ (s⁻¹) to1×10⁴ (s⁻¹) and a non-radiation rate constant (k_(nr)) thereof is 1×10⁷(s⁻¹) to 1×10⁸ (s⁻¹). Accordingly, the phosphorescence quantumefficiency (φ_(p)) can be estimated to be 1×10⁻³ to 1×10⁻⁵.

Here, when the phosphorescence quantum efficiency (φ_(p)) is 1×10⁻⁴ andx is 100, the energy transfer efficiency (φ_(ET)) is 1.0%. If x is 1000,the energy transfer efficiency (φ_(ET)) is 9.1%. Note that there is apositive correlation between x and an absorption coefficient; thus, asthe absorption coefficient is increased, x is also increased. That is,even in the case where the phosphorescence quantum efficiency (φ_(p)) ofthe molecule on a donor side (the molecule B in FIGS. 1A to 1D) is low,if the absorption coefficient of the molecule on an acceptor side (themolecule A in FIGS. 1A to 1D) is high, the energy transfer by theFörster mechanism can occur.

As described above, energy transfer between triplet excitons can partlyoccur by the Förster mechanism. Thus, here, the energy transfer throughTTA by not only the Dexter mechanism but also that by the Förstermechanism are considered.

In the case where the energy transfer by the Förster mechanism iscaused, as shown in the following Formula (3), generally, the absorptioncoefficient of the molecule is high when the oscillator strength (f) ofthe molecule is large.

f=4.32×10⁻⁹∫ε({tilde over (v)})d{tilde over (v)}  (3)

f: oscillator strength, ε: absorption coefficient

Hence, a molecular design is performed using quantum chemicalcalculations so that the oscillator strength (f) between the tripletexcited state (T₁) that is the lowest level and the triplet excitedstate (T_(n)) that is higher than the T₁ is increased. However, in thecase where there is a plurality of triplet excited states (T_(n)) thatis higher than T₁, the total of the oscillator strengths in the tripletexcited states is considered to be the oscillator strength (f). Notethat by the molecular design, it is found that the oscillator strength(f) of the molecule is increased when a compound including an anthraceneskeleton is used. Structural formulae of the compounds each including ananthracene skeleton are shown below.

The quantum chemical calculation method of the above compound is asfollows. Note that Gaussian 09 is used as the quantum chemistrycomputational program. A high performance computer (ICE X manufacturedby SGI Japan, Ltd.) is used for the calculation.

First, stable structures and electron states in the singlet ground state(S₀) and the T₁ state are calculated using the density functional theory(DFT). After that, vibration analysis is conducted, and the T₁ level iscalculated from the energy difference between the stable structures inthe S₀ state and in the T₁ state. As a basis function, 6-311G (d,p) isused. As a functional, B3LYP is used. In the DFT, the total energy ofthe molecules is represented as the sum of potential energy,electrostatic energy between electrons, electronic kinetic energy, andexchange-correlation energy including all the complicated interactionsbetween electrons. Also in the DFT, an exchange-correlation interactionis approximated by a functional (a function of another function) of oneelectron potential represented in terms of electron density; thus,electron states can be obtained with high accuracy.

Next, a time-dependent density functional theory (TD-DFT) is used tocalculate the transition dipole moment and the oscillator strength (f)which relate to the transition from the T₁ level to the T_(n) level. Asa basis function, 6-311G(d,p) is used, and as a functional, CAM-B3LYP isused. In the calculation using TD-DFT, stable structures and electronstates in the T₁ state obtained from the calculation using CAM-B3LYP asa functional of DFT are used.

Note that a T_(n) state in TD-DFT indicates a triplet excited state atan energy level lower than a value obtained by adding 0.6 eV toexcitation energy corresponding to twice the T₁ level by the calculationusing TD-DFT. However, even if an excited state satisfying the abovecondition is included in a portion other than an anthracene skeleton inthe compound including the anthracene skeleton, it is excluded from theT_(n) state because of being not involved in triplet excitation of theentire compound.

From the calculation using TD-DFT, the T₁ level of 1,5CzP2A is 1.67 eVand the T₁ level of 1,8CzP2A is 1.66 eV. From the calculation usingTD-DFT, it is found that there are two triplet excited statescorresponding to the T_(n) levels where excitation energy from the T₁level is less than a value obtained by adding 0.6 eV to the T₁ level ineach of 1,5CzP2A and 1,8CzP2A. Note that excitation energies from the T₁level to the T_(n) level in 1,5CzP2A are 1.80 eV and 2.07 eV, andexcitation energies from the T₁ level to the T_(n) level in 1.8CzP2A was1.81 eV and 2.06 eV.

FIG. 2 shows the directions of the transition dipole moment between theT₁ level to the T_(n) level and the oscillator strengths (f) of each of1,5CzP2A and 1,8CzP2A, obtained from the calculation using TD-DFT. Notethat in the molecular arrangement of the molecules in FIG. 2, the majoraxis of the anthracene skeleton is aligned with the x-axis and the minoraxis is aligned with the y-axis.

As shown in FIG. 2, the transition dipole moment of 1,8CzP2A is formedmainly using components in the X-axis direction (an arrow “a” in FIG.2); the transition dipole moment of 1,5CzP2A is formed using componentsin the X-axis direction and Y-axis direction (an arrow “b” in FIG. 2).Note that the oscillator strength (f) of 1,8CzP2A is calculated to be0.0020, and the oscillator strength (f) of 1,5CzP2A is calculated to be0.0032. The results reveals that the oscillator strength (f) of 1,5CzP2Ais larger than that of 1,8CzP2A, and the transition between the T₁ leveland the T_(n) level more easily occurs in 1,5CzP2A than in 1,8CzP2A.That is, 1,5CzP2A has a higher probability of TTA caused by energytransfer by the Förster mechanism than 1,8CzP2A.

The magnitude of the transition dipole moment and the oscillatorstrength (f) have a relation shown in Formulae (4) in which theoscillator strength (f) is proportional to the square of the magnitudeof the transition dipole moment.

$\begin{matrix}{{f = \frac{{\mu_{m\; n}}^{2}}{{\mu_{0}}^{2}}}{{\mu_{0}}^{2} = \frac{3{he}^{2}}{8\pi \; {mv}}}} & (4)\end{matrix}$

f: oscillator strength, μ_(mn): transition dipole moment, ,μ₀:oscillation electric dipole moment, h: Planck constant, e: quantum ofelectricity, m: mass of electrons, v: wave number

In each of 1,5CzP2A and 1,8CzP2A, the compound is divided into ananthracene skeleton 501 and carbazole skeletons 502 as units (skeletons)constituting the compound, and the transition dipole moment in thetransition from the T₁ level to the T_(n) level in each of the units isanalyzed. Here, only the transition between the main molecular orbits ofthe transition with the largest oscillator strength among thetransitions from the T₁ level to the T_(n) level, is analyzed. FIG. 3shows the results.

From the results in FIG. 3, in 1,8CzP2A, the components of the twocarbazole skeletons 502 in the y-axis direction of the transition dipolemoment are in the direction opposite to each other, thereby weakeningthe components in the y-axis direction for each other. In 1,5CzP2A, thecomponents of the two carbazole skeletons 502 in the y-axis direction ofthe transition dipole moment are in the same direction, therebystrengthening the components the y-axis direction for each other. As aresult, in the entire 1,5CzP2A, the magnitude of the transition dipolemoment in the y-axis direction derived from the carbazole skeletons 502is large. Thus, as shown in the above Formulae (4), it is found that theoscillator strength (f) of 1,5CzP2A having a larger transition dipolemoment is larger than that of 1,8CzP2A. That is, the following can besaid, also in view of the molecular structure, that the oscillatorstrength (f) of 1,5CzP2A is larger than that of 1,8CzP2A, the transitionbetween the T₁ level and the T_(n) level more easily occurs in 1,5CzP2Athan in 1,8CzP2A, and thus 1,5CzP2A has a higher probability of TTAcaused by energy transfer by the Förster mechanism than 1,8CzP2A.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 2

In this embodiment, an organic compound of one embodiment of the presentinvention will be described. The organic compound described in thisembodiment is an organic compound in which the probability of TTA causedby energy transfer by the Förster mechanism can be increased, as in1,5CzP2A and 1,8CzP2A described as examples in Embodiment 1.

The organic compound described in this embodiment has a feature in thata carbazole skeleton is bonded to an anthracene skeleton directly orthrough an arylene group. The organic compound described in thisembodiment is an organic compound having a structure represented byGeneral Formula (G1).

Note that either of A¹ and A² in General Formula (G1) is a substituentrepresented by General Formula (G1-1), and the other is hydrogen oranother substituent. That is, α² is bonded to the 5-position or8-position of the anthracene skeleton in General Formula (G1). Inaddition, α¹ and α² individually represent a substituted orunsubstituted phenylene group. Furthermore, n and m individuallyrepresent 1 or 2. Furthermore, General Formula (G1) may include one ormore substituents. When General Formula (G1) includes the one or moresubstituents and either of A¹ and A² is the another substituent, theyindependently represent an alkyl group having 1 to 6 carbon atoms, acycloalkyl group having 1 to 6 carbon atoms, an alkylphenyl group, or aphenyl group.

Note that as the substitution site of General Formula (G1-1), A¹ ispreferred to A² in General Formula (G1) because the total of theoscillator strengths for excitation from the T₁ level to the T_(n) leveltends to be larger.

Furthermore, as the substitution site of General Formula (G1-1), A¹ ispreferred to A² because the two substituents each including thecarbazole skeleton are bonded to a 1-position and a 5-position of theanthracene skeleton, and steric repulsion of the two substituents eachincluding the carbazole skeleton is prevented. Similarly, when GeneralFormula (G1) has other substituents, it is preferable that thesubstituents be provided so as not to be adjacent (e.g., at the 1- and2-positions, the 2- and 3-positions, and the 1- and 9-positions) to eachother at the same time because the steric repulsion can be prevented.

Note that in General Formulae (G1) and (G1-1), examples of phenylenegroups represented by α¹ and α² include a para-phenylene group, ameta-phenylene group, and an orthophenylene group.

Specific examples of the phenylene groups represented by α¹ and α² inGeneral Formulae (G1) and (G1-1) are represented by Structural Formulae(α-1) to (α-5).

In the phenylene groups represented by α¹ and α² in General Formulae(G1) and (G1-1), the substitution sites of the carbazole skeleton andthe anthracene skeleton can be any positions of a para-position, ameta-position, and an ortho-position. When the phenylene group has thesubstituents at the para-position, a high carrier-transport property isobtained, which is preferable. When the phenylene group has thesubstituents at the meta-position, a bulky structure is obtained andthus evaporation temperature can be low, which is preferable.

In the case where the phenylene groups represented by α¹ and α² inGeneral Formulae (G1) and (G1-1) further have substituents, examples ofthe substituents include an alkyl group having 1 to 6 carbon atoms and acycloalkyl group having 1 to 6 carbon atoms. Specific examples include amethyl group, an ethyl group, a propyl group, an isopropyl group, abutyl group, a tert-butyl group, a pentyl group, an isopentyl group, ahexyl group, an isohexyl group, and a cyclohexyl group.

Furthermore, specific examples of the substituents of the phenylenegroups represented by α¹ and α² in General Formulae (G1) and (G1-1) arerepresented by Structural Formulae (R-1) to (R-11).

In the case where the phenylene groups represented by α¹ and α² inGeneral Formulae (G1) and (G1-1) have the substituents, improvement insolubility and improvement in thermophysical property can be expected,which is preferable. Meanwhile, in the case where the phenylene groupsrepresented by α¹ and α² in the General Formulae (G1) and (G1-1) do nothave the substituents, synthesis is performed easily, which ispreferable.

A structure of another organic compound is represented by GeneralFormula (G2).

Note that either of A¹ and A² in General Formula (G2) is a substituentrepresented by General Formula (G2-1), and the other is hydrogen oranother substituent. Furthermore, General Formula (G2) may include oneor more substituents. When General Formula (G2) includes the one or moresubstituents and either of A¹ and A² is the another substituent, theyindependently represent an alkyl group having 1 to 6 carbon atoms, acycloalkyl group having 1 to 6 carbon atoms, an alkylphenyl group, or aphenyl group.

Specific examples of the structural formulae of the above-describedorganic compound are shown below. Note that the present invention is notlimited to these examples.

Examples of a method for synthesizing the organic compound representedby General Formula (G1) and General Formula (G1-1) are describedreferring to Synthesis Schemes (F1-1) and (F1-2). That is, byapplication of coupling reactions shown in Synthesis Schemes (F1-1) and(F1-2), the organic compound represented by General Formula (G1) andGeneral Formula (G1-1) can be synthesized. As in the above case, eitherof A¹ and A² in General Formula (G1) is the substituent represented byGeneral Formula (G1-1), and the other is hydrogen or anothersubstituent.

In Synthesis Schemes (F1-1) and (F1-2), either of X¹ and X² and X³represent halogen. Specifically, iodine, bromine, and chlorine havehigher reactivity in this order and are preferred in this order. B¹ andB² each represent a boron compound; specifically, represent boronic acidor alkoxy boron. Note that an aryl aluminum compound, an aryl zirconiumcompound, an aryl zinc compound, an aryl tin compound, or the like mayalso be used. In addition, α¹ and α² individually represent asubstituted or unsubstituted phenylene group. Note that n and mindividually represent 1 or 2.

There are a variety of reaction conditions for the coupling reactions inSynthesis Schemes (F1-1) and (F1-2). As an example, a synthesis methodusing a metal catalyst in the presence of a base, such as aSuzuki-Miyaura reaction, can be employed. In the above synthesis method,the synthesis is carried out in two steps of a synthesis steprepresented by Synthesis Scheme (F1-1) and a synthesis step representedby Synthesis Scheme (F1-2). However, in the case where a¹ in a compound(a2) and α² in a compound (a4) are the same and n in the compound (a2)and m in the compound (a4) are the same, two or more equivalents of thecompound (a2) may be added to the compound (a1), in which case theorganic compound can be easily synthesized in one step.

Although the example of a method for synthesizing the organic compoundis described above, the present invention is not limited thereto and anyother synthesis method may be employed.

The above-described organic compound can be used alone or in combinationwith a light-emitting substance (guest), another organic compound, orthe like in a light-emitting element.

In addition, the above-described organic compound can be used in anorganic thin film solar cell. More specifically, the organic compoundcan be used in a carrier-transport layer or a carrier-injection layersince the organic compound has a carrier-transport property. Inaddition, a mixed layer of the organic compound and an acceptorsubstance can be used as a charge generation layer. The organic compoundcan be photoexcited and hence can be used for a power generation layer.

In Embodiment 2, one embodiment of the present invention has beendescribed. Other embodiments of the present invention are described inEmbodiments 3 to 12 as follows. Note that one embodiment of the presentinvention is not limited to the above examples. That is, since variousembodiments of the present invention are disclosed in this embodimentand other embodiments, one embodiment of the present invention is notlimited to a specific embodiment.

Embodiment 3

In Embodiment 3, a light-emitting element of the present invention willbe described with reference to FIGS. 4A and 4B.

In the light-emitting element described in this embodiment, an EL layer102 including a light-emitting layer 113 is provided between a pair ofelectrodes (a first electrode (anode) 101 and a second electrode(cathode) 103). The EL layer 102 includes, in addition to thelight-emitting layer 113, a hole-injection layer 111, a hole-transportlayer 112, an electron-transport layer 114, an electron-injection layer115, and the like.

When a voltage is applied to the light-emitting element, holes injectedfrom the first electrode 101 side and electrons injected from the secondelectrode 103 side recombine in the light-emitting layer 113; withenergy generated by the recombination, a light-emitting substance suchas an organometallic complex that is contained in the light-emittinglayer 113 emits light.

The hole-injection layer 111 in the EL layer 102 can inject holes intothe hole-transport layer 112 or the light-emitting layer 113 and can beformed of, for example, a substance having a high hole-transportproperty and a substance having an acceptor property, in which caseelectrons are extracted from the substance having a high hole-transportproperty by the substance having an acceptor property to generate holes.Thus, holes are injected from the hole-injection layer 111 into thelight-emitting layer 113 through the hole-transport layer 112. For thehole-injection layer 111, a substance having a high hole-injectionproperty can also be used. For example, molybdenum oxide, vanadiumoxide, ruthenium oxide, tungsten oxide, manganese oxide, or the like canbe used. Alternatively, the hole-injection layer 111 can be formed usinga phthalocyanine-based compound such as phthalocyanine (abbreviation:H₂Pc) and copper phthalocyanine (CuPc), an aromatic amine compound suchas 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl(abbreviation: DPAB) andN,N′-bis{4-[bis(3-methylphenyl)amino]phenyl}-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine(abbreviation: DNTPD), or a high molecular compound such aspoly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid)(abbreviation: PEDOT/PSS).

A preferred specific example in which the light-emitting elementdescribed in this embodiment is fabricated is described below.

For the first electrode (anode) 101 and the second electrode (cathode)103, a metal, an alloy, an electrically conductive compound, a mixturethereof, and the like can be used. Specific examples are indiumoxide-tin oxide (indium tin oxide), indium oxide-tin oxide containingsilicon or silicon oxide, indium oxide-zinc oxide (indium zinc oxide),indium oxide containing tungsten oxide and zinc oxide, gold (Au),platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum(Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), and titanium(Ti). In addition, an element belonging to Group 1 or Group 2 of theperiodic table, for example, an alkali metal such as lithium (Li) orcesium (Cs), an alkaline earth metal such as calcium (Ca) or strontium(Sr), magnesium (Mg), and an alloy containing such an element (MgAg orAlLi); a rare earth metal such as europium (Eu) or ytterbium (Yb) and analloy containing such an element; a graphene compound such as grapheneor graphene oxide; and the like can be used. The first electrode (anode)101 and the second electrode (cathode) 103 can be formed by, forexample, a sputtering method or an evaporation method (including avacuum evaporation method).

As the substance having a high hole-transport property which is used forthe hole-injection layer 111 and the hole-transport layer 112, any of avariety of organic compounds such as aromatic amine compounds, carbazolederivatives, aromatic hydrocarbons, and high molecular compounds (e.g.,oligomers, dendrimers, or polymers) can be used. The organic compoundused for the composite material is preferably an organic compound havinga high hole-transport property. Specifically, a substance having a holemobility of 1×10⁻⁶ cm²/Vs or more is preferably used. The layer formedusing the substance having a high hole-transport property is not limitedto a single layer and may be formed by stacking two or more layers.Organic compounds that can be used as the substance having ahole-transport property are specifically given below.

The substance used for the hole-transport layer 112 preferably hashigher S₁ and T₁ levels than the light-emitting layer 113 that isadjacent to the hole-transport layer 112 because diffusion of excitationenergy to the hole-transport layer 112 can be suppressed. Furthermore,the substance used for the hole-transport layer 112 preferably has ahigher LUMO level (a larger value) than the light-emitting layer 113that is adjacent to the hole-transport layer 112 because passage ofelectrons through the light-emitting layer 113 to the hole-transportlayer 112 can be suppressed. Furthermore, a HOMO level of the substanceused for the hole-transport layer 112 is preferably deeper (a smallervalue) than or substantially equal to the HOMO level of thelight-emitting layer 113 that is adjacent to the hole-transport layer112 because easier hole-injection into the light-emitting layer 113 canbe achieved. Organic compounds that can be used as the substance havinga hole-transport property are specifically given below.

Examples of the aromatic amine compounds areN,N′-di(p-tolyl)-N,N′-diphenyl-p-phenylenediamine (abbreviation:DTDPPA), 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl(abbreviation: DPAB), DNTPD,1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B), 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl(abbreviation: NPB or α-NPD),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(carbazol-9-yl)triphenylamine(abbreviation: TCTA), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine(abbreviation: TDATA), 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (abbreviation: MTDATA),and 4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), and the like.

Specific examples of carbazole derivatives are3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2),3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1), and the like. Other examples are4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB),9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA),1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene, and thelike.

Examples of aromatic hydrocarbons are2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA),2-tert-butyl-9,10-di(1-naphthyl)anthracene,9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA),2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA),9,10-di(2-naphthyl)anthracene (abbreviation: DNA),9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene(abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA),2-tert-butyl-9,10-bis[2-(1-naphthyl)phenyl]anthracene, 9,10-bis[2-(1-naphthyl)phenyl]anthracene,2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene,2,3,6,7-tetramethyl-9,10-di(2-naphthypanthracene, 9,9′-bianthryl,10,10′-diphenyl-9,9′-bianthryl,10,10′-bis(2-phenylphenyl)-9,9′-bianthryl,10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene,tetracene, rubrene, perylene, 2,5,8,11-tetra(tert-butyl)perylene, andthe like. Besides, pentacene, coronene, or the like can also be used.The aromatic hydrocarbon which has a hole mobility of 1×10⁻⁶ cm²/Vs ormore and which has 14 to 42 carbon atoms is particularly preferable. Thearomatic hydrocarbons may have a vinyl skeleton. Examples of thearomatic hydrocarbon having a vinyl group are4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation: DPVBi),9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene (abbreviation: DPVPA),and the like.

A high molecular compound such as poly(N-vinylcarbazole) (abbreviation:PVK), poly(4-vinyltriphenylamine) (abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), orpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation:Poly-TPD) can also be used.

Examples of the substance having an acceptor property which is used forthe hole-injection layer 111 and the hole-transport layer 112 arecompounds having an electron-withdrawing group (a halogen group or acyano group) such as7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ), chloranil, and2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (HAT-CN). Inparticular, a compound in which electron-withdrawing groups are bondedto a condensed aromatic ring having a plurality of heteroatoms, likeHAT-CN, is thermally stable and preferable. Oxides of metals belongingto Groups 4 to 8 of the periodic table can be given. Specifically,vanadium oxide, niobium oxide, tantalum oxide, chromium oxide,molybdenum oxide, tungsten oxide, manganese oxide, and rhenium oxide arepreferable because of their high electron-accepting properties. Amongthese, molybdenum oxide is especially preferable because it is stable inthe air, has a low hygroscopic property, and is easy to handle.

The light-emitting layer 113 is a layer containing a light-emittingsubstance (guest material). Examples of the light-emitting substanceinclude a light-emitting substances that convert singlet excitationenergy into luminescence and light-emitting substances that converttriplet excitation energy into luminescence. In the case of a structurein which triplet excitons are converted into singlet excitons by TTA sothat emission efficiency of the singlet excitons is improved asdescribed in Embodiment 1, a light-emitting substance that convertssinglet excitation energy into luminescence is preferably used. As anexample of the light-emitting substance that converts singlet excitationenergy into luminescence, a substance that emits fluorescence(fluorescent compound) can be given.

Examples of the substance that emits fluorescence areN,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA),4-(9H-carbazol-9-yl)-4′-(9,10-diphenyl-2-anthryl)triphenylamine(abbreviation: 2YGAPPA),N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA), perylene, 2,5,8,11-tetra(tert-butyl)perylene(abbreviation: TBP),4-(10-phenyl-9-anthryl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBAPA),N,N″-(2-tert-butylanthracene-9,10-diyldi-4,1-phenylene)bis[N,N′,N′-triphenyl-1,4-phenylenediamine] (abbreviation: DPABPA),N,9-diphenyl-N-[4-(9,10-diphenyl-2-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: 2PCAPPA),N-[4-(9,10-diphenyl-2-anthryl)phenyl]-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPPA),N,N,N′,N′,N″,N″,N′″,N′″-octaphenyldibenzo[g,p]chrysene-2,7,10,15-tetraamine(abbreviation: DBC1), coumarin 30,N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCABPhA),N-(9,10-diphenyl-2-anthryl)-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPABPhA),9,10-bis(1,1′-biphenyl-2-yl)-N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylanthracen-2-amine(abbreviation: 2YGABPhA), N,N,9-triphenylanthracen-9-amine(abbreviation: DPhAPhA), coumarin 545T, N,N′-diphenylquinacridone(abbreviation: DPQd), rubrene,5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation: BPT),2-(2-{2-[4-(dimethylamino)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile(abbreviation: DCM1),2-{2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCM2),N,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (abbreviation:p-mPhTD),7,14-diphenyl-N,N,N′,N′-tetrakis(4-methylphenyl)acenaphtho[1,2-α]fluoranthene-3,10-diamine(abbreviation: p-mPhAFD),2-{2-isopropyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTI),2-{2-tert-butyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTB),2-(2,6-bis{2-[4-(dimethylamino)phenyl]ethenyl}-4H-pyran-4-ylidene)propanedinitrile(abbreviation: BisDCM),2-{2,6-bis[2-(8-methoxy-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: BisDCJTM), and the like.

In the case where the light-emitting substance that converts tripletexcitation energy into luminescence is used in the light-emitting layer113, it is preferable that an organic compound (a host material) usedwith a light-emitting substance (referred to as a dopant or a guestmaterial) have a higher probability of TTA. Specifically, an organiccompound where the total of oscillator strengths (f) for transitionsfrom the T₁ level of the organic compound to some T_(n) levels which areeach higher than the T₁ level by less than 0.6 eV is 0.0015 or more,preferably 0.0020 or more is preferably used. That is, an organiccompound in which the transition between the T₁ level and the T_(n)level easily occurs, and which has a higher probability of TTA caused byenergy transfer by the Förster mechanism, is preferably used. As anexample of the organic compound which has a higher probability of TTA,the organic compound described in Embodiment 2 can be used.

In the case where the light-emitting substance that converts tripletexcitation energy into luminescence is used in the light-emittingelement 113, it seems that when the organic compound (the host material)used with the light-emitting substance (the dopant) is designed suchthat the T₁ level of the organic compound (the host material) is thelowest, the triplet excitation energy is collected at the host materialand thus the probability of TTA is increased.

Furthermore, in the light-emitting layer 113 of the light-emittingelement in this embodiment, not only a structure in which a substancethat emits fluorescence (a fluorescence compound) is used for alight-emitting substance by utilizing TTA, but also a structure in whicha light-emitting substance that converts triplet excitation energy intoluminescence can be used together with the substance that emitsfluorescence (a fluorescence compound). Examples of the light-emittingsubstance that converts triplet excitation energy into luminescenceinclude a substance which emits phosphorescence (a phosphorescentcompound) and a thermally activated delayed fluorescent (TADF) materialwhich emits thermally activated delayed fluorescence. Note that “delayedfluorescence” exhibited by the TADF material refers to light emissionhaving the same spectrum as normal fluorescence and an extremely longlifetime. The lifetime is 1×10⁻⁶ seconds or longer, preferably 1×10⁻³seconds or longer.

Examples of the substance that emits phosphorescence are bis{2-[3′,5′-bis(trifluoromethyl)phenyl]pyridinato-N,C^(2′)}iridium(III)picolinate (abbreviation: [Ir(CF₃ppy)₂(pic)],bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)acetylacetonate (abbreviation: FIracac),tris(2-phenylpyridinato)iridium(III) (abbreviation: [Ir(ppy)₃]),bis(2-phenylpyridinato)iridium(III) acetylacetonate (abbreviation:[Ir(ppy)₂(acac)]), tris(acetylacetonato)(monophenanthroline)terbium(III)(abbreviation: [Tb(acac)₃(Phen)]), bis(benzo[h]quinolinato)iridium(III)acetylacetonate (abbreviation: [Ir(bzq)₂(acac)]),bis(2,4-diphenyl-1,3-oxazolato-N,C^(2′))iridium(III) acetylacetonate(abbreviation: [Ir(dpo)₂(acac)]),bis{2-[4′-(perfluorophenyl)phenyl]pyridinato-N,C^(2′)}iridium(III)acetylacetonate (abbreviation: [Ir(p-PF-ph)₂(acac)]),bis(2-phenylbenzothiazolato-N,C^(2′))iridium(III) acetylacetonate(abbreviation: [Ir(bt)₂(acac)]),bis[2-(2′-benzo[4,5-α]thienyl)pyridinato-N,C^(3′)]iridium(III)acetylacetonate (abbreviation: [Ir(btp)₂(acac)]),bis(1-phenylisoquinolinato-N,C^(2′))iridium(III) acetylacetonate(abbreviation: [Ir(piq)₂(acac)]),(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: [Ir(Fdpq)₂(acac)]),(acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III)(abbreviation: [Ir(mppr-Me)₂(acac)]),(acetylacetonato)bis(5-isopropyl-3-methyl-2-phenylpyrazinato)iridium(III)(abbreviation: [Ir(mppr-iPr)₂(acac)]),(acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III)(abbreviation: [Ir(tppr)₂(acac)]),bis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III)(abbreviation: [Ir(tppr)₂(dpm)],(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(abbreviation: [Ir(tBuppm)₂(acac)]),(acetylacetonato)bis(4,6-diphenylpyrirnidinato)iridium(III)(abbreviation: [Ir(dppm)₂(acac)]),2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II)(abbreviation: PtOEP),tris(1,3-diphenyl-1,3-propanedionato)(monophenanthroline)europium(III)(abbreviation: [Eu(DBM)₃(Phen)]),tris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III)(abbreviation: [Eu(TTA)₃(Phen)]), and the like.

Examples of the TADF material are fullerene, a derivative thereof, anacridine derivative such as proflavine, eosin, and the like. Otherexamples are a metal-containing porphyrin, such as a porphyrincontaining magnesium (Mg), zinc (Zn), cadmium (Cd), tin (Sn), platinum(Pt), indium (In), or palladium (Pd). Examples of the metal-containingporphyrin are a protoporphyrin-tin fluoride complex (abbreviation:SnF₂(Proto IX)), a mesoporphyrin-tin fluoride complex (abbreviation:SnF₂(Meso IX)), a hematoporphyrin-tin fluoride complex (abbreviation:SnF₂(Hemato IX)), a coproporphyrin tetramethyl ester-tin fluoridecomplex (abbreviation: SnF₂(Copro III-4Me)), an octaethylporphyrin-tinfluoride complex (abbreviation: SnF₂(OEP)), an etioporphyrin-tinfluoride complex (abbreviation: SnF₂(Etio I)), anoctaethylporphyrin-platinum chloride complex (abbreviation: PtCl₂OEP),and the like. Alternatively, a heterocyclic compound including aπ-electron rich heteroaromatic ring and a π-electron deficientheteroaromatic ring can be used, such as2-(biphenyl-4-yl)-4,6-bis(12-phenylindolo[2,3-α]carbazol-11-yl)-1,3,5-triazine(abbreviation: PIC-TRZ). Note that a material in which the π-electronrich heteroaromatic ring is directly bonded to the π-electron deficientheteroaromatic ring is particularly preferably used because the donorproperty of the π-electron rich heteroaromatic ring and the acceptorproperty of the π-electron deficient heteroaromatic ring are bothincreased and the energy difference between the S1 level and the T1level becomes small.

When the light-emitting substance that converts triplet excitationenergy into luminescence is used in the light-emitting layer 113, otherthan a structure in which one kind of organic compound (host material)is used in addition to the light-emitting substance, the followingstructure may be employed: a structure where two kinds of organiccompounds (the two kinds of organic compounds may include the above hostmaterial) that can form an excited complex (also called an exciplex) atthe time of recombination of carriers (electrons and holes) in thelight-emitting layer 113 are contained in addition to the light-emittingsubstance. In order to form an excited complex efficiently, it isparticularly preferable to combine a compound which easily acceptselectrons (a material having an electron-transport property) and acompound which easily accepts holes (a material having a hole-transportproperty). In the case where the combination of a material having anelectron-transport property and a material having a hole-transportproperty is used as a host material which forms an excited complex asdescribed above, the carrier balance between holes and electrons in thelight-emitting layer can be easily optimized by adjustment of themixture ratio of the material having an electron-transport property andthe material having a hole-transport property. The optimization of thecarrier balance between holes and electrons in the light-emitting layercan prevent a region in which electrons and holes are recombined fromexisting on one side in the light-emitting layer. By preventing theregion in which electrons and holes are recombined from existing to oneside, the reliability of the light-emitting element can be improved.

As the material having an electron-transport property that is preferablyused to form the above excited complex, a π-electron deficientheteroaromatic compound such as a nitrogen-containing heteroaromaticcompound, a metal complex, or the like can be used. Specific examplesinclude a metal complex such asbis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato) (4-phenylphenolato)aluminum(III)(abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), orbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ); aheterocyclic compound having an polyazole skeleton such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ),1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation:CO11), 2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), or2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole(abbreviation: mDBTBIm-II); a heterocyclic compound having a diazineskeleton such as 2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II),2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II),2-[3′-(9H-carbazol-9-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mCzBPDBq),2-[4-(3,6-diphenyl-9H-carbazol-9-yl)phenyl]dibenzo [f,h]quinoxaline(abbreviation: 2CzPDBq-III), 7-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 7mDBTPDBq-II);6-[3-(dibenzothiophen-4-yl)phenyl]dibenzo [f,h]quinoxaline(abbreviation: 6mDBTPDBq-II),4,6-bis[3-(phenanthren-9-yl)phenyl]pyrimidine (abbreviation:4,6mPnP2Pm), 4,6-bis[3-(4-dibenzothienyl)phenyl]pyrimidine(abbreviation: 4,6mDBTP2Pm-II), or4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine (abbreviation:4,6mCzP2Pm); a heterocyclic compound having a triazine skeleton such as2-{4-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}-4,6-diphenyl-1,3,5-triazine(abbreviation: PCCzPTzn); and a heterocyclic compound having a pyridineskeleton such as 3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine(abbreviation: 35DCzPPy) or 1,3,5-tri [3-(3-pyridyl)phenyl]benzene(abbreviation: TmPyPB). Among the above materials, heterocycliccompounds having diazine skeletons and triazine skeletons andheterocyclic compounds having pyridine skeletons have high reliabilityand are thus preferable. Heterocyclic compounds having diazine(pyrimidine or pyrazine) skeletons and triazine skeletons have anexcellent electron-transport property and contribute to a decrease indrive voltage.

As the compound that is preferably used to form the above excitedcomplex, a π-electron rich heteroaromatic compound (e.g., a carbazolederivative or an indole derivative), an aromatic amine compound, or thelike can be favorably used. Specific examples are compounds havingaromatic amine skeletons, such as2-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]spiro-9,9′-bifluorene(abbreviation: PCASF),4,4′,4″-tris[N-(1-naphthyl)-N-phenylamino]triphenylamine (abbreviation:1′-TNATA),2,7-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-spiro-9,9′-bifluorene(abbreviation: DPA2SF),N,N′-bis(9-phenylcarbazol-3-yl)-N,N′-diphenylbenzene-1,3-diamine(abbreviation: PCA2B),N-(9,9-dimethyl-2-diphenylamino-9H-fluoren-7-yl)diphenylamine(abbreviation: DPNF),N,N′,N″-triphenyl-N,N′,N″-tris(9-phenylcarbazol-3-yl)benzene-1,3,5-triamine(abbreviation: PCA3B),2-[N-(4-diphenylaminophenyl)-N-phenylamino]spiro-9,9′-bifluorene(abbreviation: DPASF),N,N′-bis[4-(carbazol-9-yl)phenyl]-N,N′-diphenyl-9,9-dimethylfluorene-2,7-diamine(abbreviation: YGA2F), NPB,N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD),4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB), BSPB, 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP), 4-phenyl-3′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: mBPAFLP),N-(9,9-dimethyl-9H-fluoren-2-yl)-N-{9,9-dimethyl-2-[N′-phenyl-N′-(9,9-dimethyl-9H-fluoren-2-yl)amino]-9H-fluoren-7-yl}phenylamine(abbreviation: DFLADFL), PCzPCA1,3-[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenyl carbazole(abbreviation: PCzDPA1),3,6-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzDPA2), DNTPD, 3,6-bis[N-(4-diphenylaminophenyl)-N-(1-naphthyl)amino]-9-phenylcarbazole(abbreviation: PCzTPN2), PCzPCA2,4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBA1BP), 4,4′-diphenyl-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBBi1BP),4-(1-naphthyl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBANB),4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBNBB),3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1),9,9-dimethyl-N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]fluoren-2-amine(abbreviation: PCBAF),N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]spiro-9,9′-bifluoren-2-amine(abbreviation: PCBASF),N-(4-biphenyl)-N-(9,9-dimethyl-9H-fluoren-2-yl)-9-phenyl-9H-carbazol-3-amine(abbreviation: PCBiF), andN-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine(abbreviation: PCBBiF); compounds having carbazole skeletons, such as1,3-bis(N-carbazolyl)benzene (abbreviation: mCP), CBP,3,6-bis(3,5-diphenylphenyl)-9-phenylcarbazole (abbreviation: CzTP), and9-phenyl-9H-3-(9-phenyl-9H-carbazol-3-yl)carbazole (abbreviation: PCCP);compounds having thiophene skeletons, such as4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II),2,8-diphenyl-4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]dibenzothiophene(abbreviation: DBTFLP-III), and4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-6-phenyldibenzothiophene(abbreviation: DBTFLP-IV); and compounds having furan skeletons, such as4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzofuran) (abbreviation: DBF3P-II)and 4-{3-[3-(9-phenyl-9H-fluoren-9-yl)phenyl]phenyl}dibenzofuran(abbreviation: mmDBFFLBi-II). Among the above materials, the compoundshaving aromatic amine skeletons and the compounds having carbazoleskeletons are preferred because these compounds are highly reliable andhave an excellent hole-transport property and contribute to a reductionin drive voltage.

In the light-emitting element, the light-emitting layer 113 does notnecessarily have the single-layer structure illustrated in FIG. 4A andmay have a stacked-layer structure including two or more layers asillustrated in FIG. 4B. In that case, each layer in the stacked-layerstructure emits light. For example, fluorescence utilizing TTA isobtained from a first light-emitting layer 113(a 1), and phosphorescenceis obtained from a second light-emitting layer 113(a 2) stacked over thefirst light-emitting layer. Note that the stacking order may bereversed. It is preferable that light emission due to energy transferfrom an excited complex to a dopant be obtained from the layer thatemits phosphorescence. The emission color of one layer and that of theother layer may be the same or different. In the case where the emissioncolors are different, a structure in which, for example, blue light fromone layer and orange, yellow light, or the like from the other layer canbe obtained can be formed. Each layer may contain various kinds ofdopants.

Note that in the case where the light-emitting layer 113 has astacked-layer structure, a light-emitting substance converting singletexcitation energy into light emission or a light-emitting substanceconverting triplet excitation energy into light emission can be usedalone or in combination, for example. In that case, the followingsubstances can be used.

The electron-transport layer 114 is a layer containing a substancehaving a high electron-transport property (also referred to as anelectron-transport compound). For the electron-transport layer 114, ametal complex such as tris(8-quinolinolato)aluminum (abbreviation:Alq₃), tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃),BeBq₂, BAlq, bis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation:Zn(BOX)₂), or bis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation:Zn(BTZ)₂) can be used. Alternatively, a heteroaromatic compound such asPBD, TAZ,3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: p-EtTAZ), bathophenanthroline (abbreviation: Bphen),bathocuproine (abbreviation: BCP), or4,4′-bis(5-methylbenzoxazol-2-yl)stilbene (abbreviation: BzOs) can alsobe used. A high molecular compound such as poly(2,5-pyridinediyl)(abbreviation: PPy),poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation: PF-Py), or poly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2′-b ipyridine-6,6′-diyl)](abbreviation: PF-BPy) can also be used. The substances listed here aremainly ones that have an electron mobility of 1×10⁻⁶ cm²/Vs or more.Note that any substance other than the substances listed here may beused for the electron-transport layer 114 as long as theelectron-transport property is higher than the hole-transport property.

The substance used for the electron-transport layer 114 preferably hashigher S₁ and T₁ levels than the light-emitting layer 113 that isadjacent to the electron-transport layer 114 because diffusion ofexcitation energy to the electron-transport layer 114 can be prevented.Furthermore, the substance used for the electron-transport layer 114preferably has a deeper HOMO level (a smaller value) than thelight-emitting layer 113 adjacent to the electron-transport layer 114because passage of holes through the light-emitting layer 113 to theelectron-transport layer 114 can be suppressed. Furthermore, a LUMOlevel of the substance used for the electron-transport layer 114 ispreferably higher (a larger value) than or substantially equal to theLUMO level of the light-emitting layer 113 that is adjacent to theelectron-transport layer 114 because easier electron-injection into thelight-emitting layer 113 can be achieved.

The electron-transport layer 114 is not limited to a single layer, butmay be a stack of two or more layers each containing any of thesubstances listed above.

The electron-injection layer 115 is a layer containing a substancehaving a high electron-injection property. For the electron-injectionlayer 115, an alkali metal, an alkaline earth metal, or a compoundthereof such as lithium fluoride (LiF), cesium fluoride (CsF), calciumfluoride (CaF₂), or lithium oxide (LiO_(x)) can be used. A rare earthmetal compound like erbium fluoride (ErF₃) can also be used. Anelectride may also be used for the electron-injection layer 115.Examples of the electride include a substance in which electrons areadded at high concentration to calcium oxide-aluminum oxide. Any of thesubstances for forming the electron-transport layer 114, which are givenabove, can be used.

A composite material in which an organic compound and an electron donor(donor) are mixed may also be used for the electron-injection layer 115.Such a composite material is excellent in an electron-injection propertyand an electron-transport property because electrons are generated inthe organic compound by the electron donor. In this case, the organiccompound is preferably a material that is excellent in transporting thegenerated electrons. Specifically, for example, the substances forforming the electron-transport layer 114 (e.g., a metal complex or aheteroaromatic compound), which are given above, can be used. As theelectron donor, a substance showing an electron-donating property withrespect to the organic compound may be used. Specifically, an alkalimetal, an alkaline earth metal, and a rare earth metal are preferable,and lithium, cesium, magnesium, calcium, erbium, ytterbium, and the likeare given. In addition, an alkali metal oxide or an alkaline earth metaloxide is preferable, and lithium oxide, calcium oxide, barium oxide, andthe like are given. A Lewis base such as magnesium oxide can also beused. An organic compound such as tetrathiafulvalene (abbreviation: TTF)can also be used.

Note that each of the hole-injection layer 111, the hole-transport layer112, the light-emitting layer 113, the electron-transport layer 114, andthe electron-injection layer 115 can be formed by any one or anycombination of the following methods: an evaporation method (including avacuum evaporation method), a printing method (such as relief printing,intaglio printing, gravure printing, planography printing, and stencilprinting), an ink-jet method, a coating method, and the like. Besidesthe above-mentioned materials, an inorganic compound such as a quantumdot or a high molecular compound (e.g., an oligomer, a dendrimer, or apolymer) may be used for the hole-injection layer 111, thehole-transport layer 112, the light-emitting layer 113, theelectron-transport layer 114, and the electron-injection layer 115,which are described above.

In the above light-emitting element, current flows due to a potentialdifference applied between the first electrode 101 and the secondelectrode 103 and holes and electrons recombine in the EL layer 102,whereby light is emitted. Then, the emitted light is extracted outsidethrough one or both of the first electrode 101 and the second electrode103. Therefore, one or both of the first electrode 101 and the secondelectrode 103 are electrodes having a light-transmitting property.

As described above, in the light-emitting element of this embodiment,the characteristics of the light-emitting element can be improved by theuse of the above-described desired structure for a light-emitting layer.Specifically, when TTA is utilized, light efficiency due to singletexcitation energy can be improved, whereby the light-emitting elementcan have higher efficiency than a conventional fluorescentlight-emitting element.

Note that the structure described in this embodiment can be used incombination with any of the structures described in the otherembodiments, as appropriate.

Embodiment 4

In this embodiment, a light-emitting element (hereinafter referred to asa tandem light-emitting element) including a plurality of EL layers isdescribed.

A light-emitting element described in this embodiment is a tandemlight-emitting element including, between a pair of electrodes (a firstelectrode 201 and a second electrode 204), a plurality of EL layers (afirst EL layer 202(1) and a second EL layer 202(2)) and acharge-generation layer 205 provided therebetween, as illustrated inFIG. 5A.

In this embodiment, the first electrode 201 functions as an anode, andthe second electrode 204 functions as a cathode. Note that the firstelectrode 201 and the second electrode 204 can have structures similarto those described in Embodiment 3. In addition, either or both of theEL layers (the first EL layer 202(1) and the second EL layer 202(2)) mayhave structures similar to those described in Embodiment 3. In otherwords, the structures of the first EL layer 202(1) and the second ELlayer 202(2) may be the same as or different from each other. When thestructures are the same, Embodiment 3 can be referred to.

The charge-generation layer 205 provided between the plurality of ELlayers (the first EL layer 202(1) and the second EL layer 202(2)) has afunction of injecting electrons into one of the EL layers and injectingholes into the other of the EL layers when a voltage is applied betweenthe first electrode 201 and the second electrode 204. In thisembodiment, when a voltage is applied such that the potential of thefirst electrode 201 is higher than that of the second electrode 204, thecharge-generation layer 205 injects electrons into the first EL layer202(1) and injects holes into the second EL layer 202(2).

Note that in terms of light extraction efficiency, the charge-generationlayer 205 preferably has a property of transmitting visible light(specifically, the charge-generation layer 205 has a visible lighttransmittance of 40% or more). The charge-generation layer 205 functionseven when it has lower conductivity than the first electrode 201 or thesecond electrode 204.

The charge-generation layer 205 may have either a structure in which anelectron acceptor (acceptor) is added to an organic compound having ahigh hole-transport property or a structure in which an electron donor(donor) is added to an organic compound having a high electron-transportproperty. Alternatively, both of these structures may be stacked.

In the case of the structure in which an electron acceptor is added toan organic compound having a high hole-transport property, as theorganic compound having a high hole-transport property, the substanceshaving a high hole-transport property which are given in Embodiment 3 asthe substances used for the hole-injection layer 111 and thehole-transport layer 112 can be used. For example, an aromatic aminecompound such as NPB, TPD, TDATA, MTDATA, or BSPB, or the like can beused. The substances listed here are mainly ones that have a holemobility of 1×10⁻⁶ cm²/Vs or more. Note that any organic compound otherthan the compounds listed here may be used as long as the hole-transportproperty is higher than the electron-transport property.

As the electron acceptor,7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ), chloranil, and the like can be given. Oxides of metalsbelonging to Groups 4 to 8 of the periodic table can also be given.Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromiumoxide, molybdenum oxide, tungsten oxide, manganese oxide, and rheniumoxide are preferable because of their high electron-acceptingproperties. Among these, molybdenum oxide is especially preferablebecause it is stable in the air, has a low hygroscopic property, and iseasy to handle.

In the case of the structure in which an electron donor is added to anorganic compound having a high electron-transport property, as theorganic compound having a high electron-transport property, thesubstances having a high electron-transport property which are given inEmbodiment 3 as the substances used for the electron-transport layer 114can be used. For example, a metal complex having a quinoline skeleton ora benzoquinoline skeleton, such as Alq, Almq₃, BeBq₂, or BAlq, or thelike can be used. Alternatively, a metal complex having an oxazole-basedligand or a thiazole-based ligand, such as Zn(BOX)₂ or Zn(BTZ)₂, can beused. Alternatively, in addition to such a metal complex, PBD, OXD-7,TAZ, Bphen, BCP, or the like can be used. The substances listed here aremainly ones that have an electron mobility of 1×10⁻⁶ cm²/Vs or more.Note that any organic compound other than the compounds listed here maybe used as long as the electron-transport property is higher than thehole-transport property.

As the electron donor, it is possible to use an alkali metal, analkaline earth metal, a rare earth metal, metals belonging to Groups 2and 13 of the periodic table, or an oxide or carbonate thereof.Specifically, lithium (Li), cesium (Cs), magnesium (Mg), calcium (Ca),ytterbium (Yb), indium (In), lithium oxide, cesium carbonate, or thelike is preferably used. Alternatively, an organic compound such astetrathianaphthacene may be used as the electron donor.

Note that forming the charge-generation layer 205 by using any of theabove materials can suppress a drive voltage increase caused by thestack of the EL layers. The charge-generation layer 205 can be formed byany one or any combination of the following methods: an evaporationmethod (including a vacuum evaporation method), a printing method (suchas relief printing, intaglio printing, gravure printing, planographyprinting, and stencil printing), an ink-jet method, a coating method,and the like.

Although the light-emitting element including two EL layers is describedin this embodiment, the present invention can be similarly applied to alight-emitting element in which n EL layers (202(1) to 202(n)) (n isthree or more) are stacked as illustrated in FIG. 5B. In the case wherea plurality of EL layers are included between a pair of electrodes as inthe light-emitting element according to this embodiment, by providingcharge-generation layers (205(1) to 205(n−1)) between the EL layers,light emission in a high luminance region can be obtained with currentdensity kept low. Since the current density can be kept low, the elementcan have a long lifetime.

When the EL layers have different emission colors, a desired emissioncolor can be obtained from the whole light-emitting element. Forexample, in a light-emitting element having two EL layers, when anemission color of the first EL layer and an emission color of the secondEL layer are complementary colors, the light-emitting element can emitwhite light as a whole. Note that “complementary colors” refer to colorsthat can produce an achromatic color when mixed. In other words, mixinglight of complementary colors allows white light emission to beobtained. Specifically, a combination in which blue light emission isobtained from the first EL layer and yellow or orange light emission isobtained from the second EL layer is given as an example. In that case,it is not necessary that both of blue light emission and yellow (ororange) light emission are fluorescence, and the both are notnecessarily phosphorescence. For example, a combination in which bluelight emission is fluorescence and yellow (or orange) light emission isphosphorescence or a combination in which blue light emission isphosphorescence and yellow (or orange) light emission is fluorescencemay be employed.

The same can be applied to a light-emitting element having three ELlayers. For example, the light-emitting element as a whole can providewhite light emission when the emission color of the first EL layer isred, the emission color of the second EL layer is green, and theemission color of the third EL layer is blue.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 5

In this embodiment, a light-emitting device will be described.

The light-emitting device may be either a passive matrix light-emittingdevice or an active matrix light-emitting device. Any of thelight-emitting elements described in other embodiments can be used inthe light-emitting device described in this embodiment.

In this embodiment, first, an active matrix light-emitting device isdescribed with reference to FIGS. 6A to 6C.

Note that FIG. 6A is a top view illustrating a light-emitting device andFIG. 6B is a cross-sectional view taken along the chain line A-A′ inFIG. 6A. The light-emitting device of this embodiment includes a pixelportion 302 provided over an element substrate 301, a driver circuitportion (a source line driver circuit) 303, and driver circuit portions(gate line driver circuits) 304 a and 304 b. The pixel portion 302, thedriver circuit portion 303, and the driver circuit portions 304 a and304 b are sealed between the element substrate 301 and a sealingsubstrate 306 with a sealant 305.

In addition, over the element substrate 301, a lead wiring 307 forconnecting an external input terminal, through which a signal (e.g., avideo signal, a clock signal, a start signal, or a reset signal) or anpotential from the outside is transmitted to the driver circuit portion303 and the driver circuit portions 304 a and 304 b, is provided. Here,an example is described in which a flexible printed circuit (FPC) 308 isprovided as the external input terminal. Although only the FPC isillustrated here, the FPC may be provided with a printed wiring board(PWB). The light-emitting device in this specification includes, in itscategory, not only the light-emitting device itself but also thelight-emitting device provided with the FPC or the PWB.

Next, a cross-sectional structure is described with reference to FIG.6B. The driver circuit portions and the pixel portion are formed overthe element substrate 301; the driver circuit portion 303 that is thesource line driver circuit and the pixel portion 302 are illustratedhere.

The driver circuit portion 303 is an example in which FETs 309 and 310are combined. Note that the driver circuit portion 303 may be formedwith a circuit including transistors having the same conductivity type(either n-channel transistors or p-channel transistors) or a CMOScircuit including an n-channel transistor and a p-channel transistor.Although this embodiment shows a driver integrated type in which thedriver circuit is formed over the substrate, the driver circuit is notnecessarily formed over the substrate, and may be formed outside thesubstrate.

The pixel portion 302 includes a switching FET (not illustrated) and acurrent control FET 312, and a wiring of the current control FET 312 (asource electrode or a drain electrode) is electrically connected to afirst electrode (anode) (313 a or 313 b) of a light-emitting element 317a or 317 b. Although the pixel portion 302 includes two kinds of FETs(the switching FET and the current control FETs 312) in this embodiment,one embodiment of the present invention is not limited thereto. Thepixel portion 302 may include, for example, three or more kinds of FETsand a capacitor in combination.

As the FETs 309, 310, and 312, for example, a staggered transistor or aninverted staggered transistor can be used. Examples of a semiconductormaterial that can be used for the FETs 309, 310, and 312 are a Group 13semiconductor, a Group 14 semiconductor (e.g., silicon), a compoundsemiconductor, an oxide semiconductor, and an organic semiconductor. Inaddition, there is no particular limitation on the crystallinity of thesemiconductor material, and an amorphous semiconductor or a crystallinesemiconductor can be used. In particular, an oxide semiconductor ispreferably used for the FETs 309, 310, and 312. Examples of the oxidesemiconductor are In—Ga oxides, In-M-Zn oxides (M is Al, Ga, Y, Zr, La,Ce, Hf, or Nd), and the like. For example, an oxide semiconductormaterial that has an energy gap of 2 eV or more, preferably 2.5 eV ormore and further preferably 3 eV or more, is used, so that the off-statecurrent of the transistors can be reduced.

In addition, conductive films (320 a and 320 b) for optical adjustmentare stacked over the first electrodes 313 a and 313 b. For example, asillustrated in FIG. 6B, in the case where the wavelengths of lightextracted from the light-emitting elements 317 a and 317 b are differentfrom each other, the thicknesses of the conductive films 320 a and 320 bare different from each other. In addition, an insulator 314 is formedto cover end portions of the first electrodes (313 a and 313 b). In thisembodiment, the insulator 314 is formed using a positive photosensitiveacrylic resin. The first electrodes (313 a and 313 b) are used as theanodes in this embodiment.

The insulator 314 preferably has a curved surface with curvature at anupper end portion or a lower end portion thereof. This enables favorablecoverage by a film to be formed over the insulator 314. The insulator314 can be formed using, for example, either a negative photosensitiveresin or a positive photosensitive resin. The material for the insulator314 is not limited to an organic compound and an inorganic compound suchas silicon oxide, silicon oxynitride, or silicon nitride can also beused.

An EL layer 315 and a second electrode 316 are stacked over the firstelectrodes (313 a and 313 b). In the EL layer 315, at least alight-emitting layer is provided. In the light-emitting elements (317 aand 317 b) including the first electrodes (313 a and 313 b), the ELlayer 315, and the second electrode 316, an end portion of the EL layer315 is covered with the second electrode 316. The structure of the ELlayer 315 may be the same as or different from the single-layerstructure and the stacked layer structure described in Embodiments 2 and3. Furthermore, the structure may differ between the light-emittingelements.

For the first electrode 313, the EL layer 315, and the second electrode316, any of the materials given in Embodiment 3 can be used. The firstelectrodes (313 a and 313 b) of the light-emitting elements (317 a and317 b) are electrically connected to the lead wiring 307 in a region321, so that an external signal is input through the FPC 308.

The second electrode 316 of the light-emitting elements (317 a and 317b) is electrically connected to a lead wiring 323 in a region 322, sothat an external signal is input through the FPC 308 although it is notillustrated.

Although the cross-sectional view in FIG. 6B illustrates only the twolight-emitting elements 317, a plurality of light-emitting elements arearranged in a matrix in the pixel portion 302. Specifically, in thepixel portion 302, light-emitting elements that emit light of two kindsof colors (e.g., B and Y), light-emitting elements that emit light ofthree kinds of colors (e.g., R, G, and B), light-emitting elements thatemit light of four kinds of colors (e.g. R, G, B, and Y) or (R, G, B,and W)), or the like are formed so that a light-emitting device capableof full color display can be obtained. In such cases, full color displaymay be achieved as follows: materials different according to theemission colors or the like of the light-emitting elements are used toform light-emitting layers (so-called separate coloring formation);alternatively, the plurality of light-emitting elements share onelight-emitting layer formed using the same material and further includecolor filters. Thus, the light-emitting elements that emit light of aplurality of kinds of colors are used in combination, so that effectssuch as an improvement in color purity and a reduction in powerconsumption can be achieved. Furthermore, the light-emitting device mayhave improved emission efficiency and reduced power consumption bycombination with quantum dots.

The sealing substrate 306 is attached to the element substrate 301 withthe sealant 305, whereby the light-emitting elements 317 a and 317 b areprovided in a space 318 surrounded by the element substrate 301, thesealing substrate 306, and the sealant 305.

The sealing substrate 306 is provided with coloring layers (colorfilters) 324, and a black layer (black matrix) 325 is provided betweenadjacent coloring layers. Note that one or both of the adjacent coloringlayers (color filters) 324 may be provided so as to partly overlap withthe black layer (black matrix) 325. Light emission obtained from thelight-emitting elements 317 a and 317 b is extracted through thecoloring layers (color filters) 324.

Note that the space 318 may be filled with an inert gas (such asnitrogen or argon) or the sealant 305. In the case where the sealant isapplied for attachment of the substrates, one or more of UV treatment,heat treatment, and the like are preferably performed.

An epoxy-based resin or glass frit is preferably used for the sealant305. The material preferably allows as little moisture and oxygen aspossible to penetrate. As the sealing substrate 306, a glass substrate,a quartz substrate, or a plastic substrate formed of fiber-reinforcedplastic (FRP), poly(vinyl fluoride) (PVF), polyester, acrylic, or thelike can be used. In the case where glass frit is used as the sealant,the element substrate 301 and the sealing substrate 306 are preferablyglass substrates for high adhesion.

Structures of the FETs electrically connected to the light-emittingelements may be different from those in FIG. 6B in the position of agate electrode; that is, the structures of FETs 326, 327, and 328 asillustrated in FIG. 6C may be employed. The coloring layer (colorfilter) 324 with which the sealing substrate 306 is provided may beprovided as illustrated in FIG. 6C such that, at a position where thecoloring layer (color filter) 324 overlaps with the black layer (blackmatrix) 325, the coloring layer (color filter) 324 further overlaps withan adjacent coloring layer (color filter) 324.

As described above, an active matrix light-emitting device can beobtained.

Note that the light-emitting device can be a passive matrixlight-emitting device as well as the above active matrix light-emittingdevice.

FIGS. 7A and 7B illustrate a passive-matrix light-emitting device. FIG.7A is a top view of the passive-matrix light-emitting device, and FIG.7B is a cross-sectional view thereof.

As illustrated in FIGS. 7A and 7B, light-emitting elements 405 includinga first electrode 402, EL layers (403 a, 403 b, and 403 c), and secondelectrodes 404 are formed over a substrate 401. Note that the firstelectrode 402 has an island-like shape, and a plurality of the firstelectrodes 402 are formed in one direction (the lateral direction inFIG. 7A) to form a striped pattern. An insulating film 406 is formedover part of the first electrode 402. A partition 407 formed using aninsulating material is provided over the insulating film 406. Thesidewalls of the partition 407 slope so that the distance between onesidewall and the other sidewall gradually decreases toward the surfaceof the substrate as illustrated in FIG. 7B.

Since the insulating film 406 has openings in part of the firstelectrode 402, the EL layers (403 a, 403 b, and 403 c) and secondelectrodes 404 which are divided as desired can be formed over the firstelectrode 402. In the example in FIGS. 7A and 7B, a mask such as a metalmask and the partition 407 over the insulating film 406 are employed toform the EL layers (403 a, 403 b, and 403 c) and the second electrodes404. In this example, the EL layers 403 a, 403 b, and 403 c emit lightof different colors (e.g., red, green, blue, yellow, orange, and white).

After the formation of the EL layers (403 a, 403 b, and 403 c), thesecond electrodes 404 are formed. Thus, the second electrode 404 isformed over the EL layers (403 a, 403 b, and 403 c) without contact withthe first electrode 402.

Note that sealing can be performed by a method similar to that used forthe active matrix light-emitting device, and description thereof is notmade.

As described above, the passive matrix light-emitting device can beobtained.

Note that in this specification and the like, a transistor or alight-emitting element can be formed using any of a variety ofsubstrates, for example. The type of a substrate is not limited to acertain type. As the substrate, a semiconductor substrate (e.g., asingle crystal substrate or a silicon substrate), an SOI substrate, aglass substrate, a quartz substrate, a plastic substrate, a metalsubstrate, a stainless steel substrate, a substrate including stainlesssteel foil, a tungsten substrate, a substrate including tungsten foil, aflexible substrate, an attachment film, paper including a fibrousmaterial, a base material film, or the like can be used, for example. Asan example of a glass substrate, a barium borosilicate glass substrate,an aluminoborosilicate glass substrate, a soda lime glass substrate, orthe like can be given. Examples of the flexible substrate, theattachment film, the base film, and the like are substrates of plasticstypified by polyethylene terephthalate (PET), polyethylene naphthalate(PEN), polyether sulfone (PES), and polytetrafluoroethylene (PTFE).Another example is a synthetic resin such as acrylic. Alternatively,polypropylene, polyester, polyvinyl fluoride, polyvinyl chloride, or thelike can be used. Alternatively, polyamide, polyimide, aramid, epoxy, aninorganic vapor deposition film, paper, or the like can be used.Specifically, the use of semiconductor substrates, single crystalsubstrates, SOI substrates, or the like enables the manufacture ofsmall-sized transistors with a small variation in characteristics, size,shape, or the like and with high current supply capability. A circuitusing such transistors achieves low power consumption of the circuit orhigh integration of the circuit.

Alternatively, a flexible substrate may be used as the substrate, and atransistor or a light-emitting element may be provided directly on theflexible substrate. Still alternatively, a separation layer may beprovided between the substrate and the transistor or the light-emittingelement. The separation layer can be used when part or the whole of asemiconductor device formed over the separation layer is separated fromthe substrate and transferred onto another substrate. In such a case,the transistor or the light-emitting element can be transferred to asubstrate having low heat resistance or a flexible substrate. For theseparation layer, a stack including inorganic films, which are atungsten film and a silicon oxide film, or an organic resin film ofpolyimide or the like formed over a substrate can be used, for example.

In other words, a transistor or a light-emitting element may be formedusing one substrate, and then transferred to another substrate. Examplesof a substrate to which a transistor or a light-emitting element istransferred are, in addition to the above-described substrates overwhich a transistor or a light-emitting element can be formed, a papersubstrate, a cellophane substrate, an aramid film substrate, a polyimidefilm substrate, a stone substrate, a wood substrate, a cloth substrate(including a natural fiber (e.g., silk, cotton, or hemp), a syntheticfiber (e.g., nylon, polyurethane, or polyester), a regenerated fiber(e.g., acetate, cupra, rayon, or regenerated polyester), or the like), aleather substrate, a rubber substrate, and the like. When such asubstrate is used, a transistor with excellent characteristics or atransistor with low power consumption can be formed, a device with highdurability or high heat resistance can be provided, or a reduction inweight or thickness can be achieved.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 6

In this embodiment, examples of a variety of electronic devices and anautomobile manufactured using a light-emitting device which is oneembodiment of the present invention are described.

Examples of the electronic device including the light-emitting deviceare television devices (also referred to as TV or television receivers),monitors for computers and the like, digital cameras, digital videocameras, digital photo frames, cellular phones (also referred to asportable telephone devices), portable game consoles, portableinformation terminals, audio playback devices, large game machines suchas pachinko machines, and the like. Specific examples of the electronicdevices are illustrated in FIGS. 8A, 8B, 8C, 8D, 8D′-1, and 8D′-2, andFIGS. 9A to 9C.

FIG. 8A illustrates an example of a television device. In the televisiondevice 7100, a display portion 7103 is incorporated in a housing 7101.The display portion 7103 can display images and may be a touch panel (aninput/output device) including a touch sensor (an input device). Notethat the light-emitting device which is one embodiment of the presentinvention can be used for the display portion 7103. In addition, here,the housing 7101 is supported by a stand 7105.

The television device 7100 can be operated by an operation switch of thehousing 7101 or a separate remote controller 7110. With operation keys7109 of the remote controller 7110, channels and volume can becontrolled and images displayed on the display portion 7103 can becontrolled. Furthermore, the remote controller 7110 may be provided witha display portion 7107 for displaying data output from the remotecontroller 7110.

Note that the television device 7100 is provided with a receiver, amodem, and the like. With the use of the receiver, general televisionbroadcasts can be received. Moreover, when the television device isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

FIG. 8B illustrates a computer, which includes a main body 7201, ahousing 7202, a display portion 7203, a keyboard 7204, an externalconnection port 7205, a pointing device 7206, and the like. Note thatthis computer can be manufactured using the light-emitting device whichis one embodiment of the present invention for the display portion 7203.The display portion 7203 may be a touch panel (an input/output device)including a touch sensor (an input device).

FIG. 8C illustrates a smart watch, which includes a housing 7302, adisplay portion 7304, operation buttons 7311 and 7312, a connectionterminal 7313, a band 7321, a clasp 7322, and the like.

The display portion 7304 mounted in the housing 7302 serving as a bezelincludes a non-rectangular display region. The display portion 7304 candisplay an icon 7305 indicating time, another icon 7306, and the like.The display portion 7304 may be a touch panel (an input/output device)including a touch sensor (an input device).

The smart watch illustrated in FIG. 8C can have a variety of functions,such as a function of displaying a variety of information (e.g., a stillimage, a moving image, and a text image) on a display portion, a touchpanel function, a function of displaying a calendar, date, time, and thelike, a function of controlling processing with a variety of software(programs), a wireless communication function, a function of beingconnected to a variety of computer networks with a wirelesscommunication function, a function of transmitting and receiving avariety of data with a wireless communication function, and a functionof reading program or data stored in a recording medium and displayingthe program or data on a display portion.

The housing 7302 can include a speaker, a sensor (a sensor having afunction of measuring force, displacement, position, speed,acceleration, angular velocity, rotational frequency, distance, light,liquid, magnetism, temperature, chemical substance, sound, time,hardness, electric field, current, voltage, electric power, radiation,flow rate, humidity, gradient, oscillation, odor, or infrared rays), amicrophone, and the like. Note that the smart watch can be manufacturedusing the light-emitting device for the display portion 7304.

FIGS. 8D, 8D′-1, and 8D′-2 illustrate an example of a cellular phone(e.g., smartphone). A cellular phone 7400 includes a housing 7401provided with a display portion 7402, a microphone 7406, a speaker 7405,a camera 7407, an external connection portion 7404, an operation button7403, and the like. In the case where a light-emitting device ismanufactured by forming a light-emitting element of one embodiment ofthe present invention over a flexible substrate, the light-emittingelement can be used for the display portion 7402 having a curved surfaceas illustrated in FIG. 8D.

When the display portion 7402 of the cellular phone 7400 illustrated inFIG. 8D is touched with a finger or the like, data can be input to thecellular phone 7400. In addition, operations such as making a call andcomposing e-mail can be performed by touch on the display portion 7402with a finger or the like.

There are mainly three screen modes of the display portion 7402. Thefirst mode is a display mode mainly for displaying an image. The secondmode is an input mode mainly for inputting data such as characters. Thethird mode is a display-and-input mode in which two modes of the displaymode and the input mode are combined.

For example, in the case of making a call or creating e-mail, acharacter input mode mainly for inputting characters is selected for thedisplay portion 7402 so that characters displayed on the screen can beinput. In this case, it is preferable to display a keyboard or numberbuttons on almost the entire screen of the display portion 7402.

When a detection device such as a gyroscope or an acceleration sensor isprovided inside the cellular phone 7400, display on the screen of thedisplay portion 7402 can be automatically changed by determining theorientation of the cellular phone 7400 (whether the cellular phone isplaced horizontally or vertically for a landscape mode or a portraitmode).

The screen modes are changed by touch on the display portion 7402 oroperation with the operation button 7403 of the housing 7401. The screenmodes can be switched depending on the kind of images displayed on thedisplay portion 7402. For example, when a signal of an image displayedon the display portion is a signal of moving image data, the screen modeis switched to the display mode. When the signal is a signal of textdata, the screen mode is switched to the input mode.

Moreover, in the input mode, if a signal detected by an optical sensorin the display portion 7402 is detected and the input by touch on thedisplay portion 7402 is not performed for a certain period, the screenmode may be controlled so as to be changed from the input mode to thedisplay mode.

The display portion 7402 may function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken by touchon the display portion 7402 with the palm or the finger, wherebypersonal authentication can be performed. In addition, by providing abacklight or a sensing light source that emits near-infrared light inthe display portion, an image of a finger vein, a palm vein, or the likecan be taken.

The light-emitting device can be used for a cellular phone having astructure illustrated in FIG. 8D′-1 or FIG. 8D′-2, which is anotherstructure of the cellular phone (e.g., a smartphone).

Note that in the case of the structure illustrated in FIG. 8D′-1 or FIG.8D′-2, text data, image data, or the like can be displayed on secondscreens 7502(1) and 7502(2) of housings 75000) and 7500(2) as well asfirst screens 7501(1) and 7501(2). Such a structure enables a user toeasily see text data, image data, or the like displayed on the secondscreens 7502(1) and 7502(2) while the cellular phone is placed in user'sbreast pocket.

Another electronic device including a light-emitting device is afoldable portable information terminal illustrated in FIGS. 9A to 9C.FIG. 9A illustrates a portable information terminal 9310 which isopened. FIG. 9B illustrates the portable information terminal 9310 whichis being opened or being folded. FIG. 9C illustrates the portableinformation terminal 9310 that is folded. The portable informationterminal 9310 is highly portable when folded. The portable informationterminal 9310 is highly browsable when opened because of a seamlesslarge display region.

A display portion 9311 is supported by three housings 9315 joinedtogether by hinges 9313. Note that the display portion 9311 may be atouch panel (an input/output device) including a touch sensor (an inputdevice). By bending the display portion 9311 at a connection portionbetween two housings 9315 with the use of the hinges 9313, the portableinformation terminal 9310 can be reversibly changed in shape from anopened state to a folded state. A light-emitting device of oneembodiment of the present invention can be used for the display portion9311. A display region 9312 in the display portion 9311 is a displayregion that is positioned at a side surface of the portable informationterminal 9310 that is folded. On the display region 9312, informationicons, file shortcuts of frequently used applications or programs, andthe like can be displayed, and confirmation of information and start ofapplication can be smoothly performed.

FIGS. 10A and 10B illustrate an automobile including a light-emittingdevice. The light-emitting device can be incorporated in the automobile,and specifically, can be included in lights 5101 (including lights ofthe rear part of the car), a wheel 5102 of a tire, part or whole of adoor 5103, or the like on the outer side of the automobile which isillustrated in FIG. 10A. The light-emitting device can also be includedin a display portion 5104, a steering wheel 5105, a gear lever 5106, aseat 5107, an inner rearview mirror 5108, or the like on the inner sideof the automobile which is illustrated in FIG. 10B, or in part of aglass window.

As described above, the electronic devices and automobiles can beobtained using the light-emitting device which is one embodiment of thepresent invention. Note that the light-emitting device can be used forelectronic devices and automobiles in a variety of fields without beinglimited to the electronic devices described in this embodiment.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 7

In this embodiment, a structure of a lighting device fabricated usingthe light-emitting element which is one embodiment of the presentinvention will be described with reference to FIGS. 11A to 11D.

FIGS. 11A to 11D are examples of cross-sectional views of lightingdevices. FIGS. 11A and 11B illustrate bottom-emission lighting devicesin which light is extracted from the substrate side, and FIGS. 11C and11D illustrate top-emission lighting devices in which light is extractedfrom the sealing substrate side.

A lighting device 4000 illustrated in FIG. 11A includes a light-emittingelement 4002 over a substrate 4001. In addition, the lighting device4000 includes a substrate 4003 with unevenness on the outside of thesubstrate 4001. The light-emitting element 4002 includes a firstelectrode 4004, an EL layer 4005, and a second electrode 4006.

The first electrode 4004 is electrically connected to an electrode 4007,and the second electrode 4006 is electrically connected to an electrode4008. In addition, an auxiliary wiring 4009 electrically connected tothe first electrode 4004 may be provided. Note that an insulating layer4010 is formed over the auxiliary wiring 4009.

The substrate 4001 and a sealing substrate 4011 are bonded to each otherby a sealant 4012. A desiccant 4013 is preferably provided between thesealing substrate 4011 and the light-emitting element 4002. Thesubstrate 4003 has the unevenness illustrated in FIG. 11A, whereby theextraction efficiency of light emitted from the light-emitting element4002 can be increased.

Instead of the substrate 4003, a diffusion plate 4015 may be provided onthe outside of the substrate 4001 as in a lighting device 4100illustrated in FIG. 11B.

A lighting device 4200 illustrated in FIG. 11C includes a light-emittingelement 4202 over a substrate 4201. The light-emitting element 4202includes a first electrode 4204, an EL layer 4205, and a secondelectrode 4206.

The first electrode 4204 is electrically connected to an electrode 4207,and the second electrode 4206 is electrically connected to an electrode4208. An auxiliary wiring 4209 electrically connected to the secondelectrode 4206 may be provided. An insulating layer 4210 may be providedunder the auxiliary wiring 4209.

The substrate 4201 and a sealing substrate 4211 with unevenness arebonded to each other by a sealant 4212. A barrier film 4213 and aplanarization film 4214 may be provided between the sealing substrate4211 and the light-emitting element 4202. The sealing substrate 4211 hasthe unevenness illustrated in FIG. 11C, whereby the extractionefficiency of light emitted from the light-emitting element 4202 can beincreased.

Instead of the sealing substrate 4211, a diffusion plate 4215 may beprovided over the light-emitting element 4202 as in a lighting device4300 illustrated in FIG. 11D.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 8

In this embodiment, examples of a lighting device which is anapplication of the light-emitting device of one embodiment of thepresent invention will be described with reference to FIG. 12.

FIG. 12 illustrates an example in which the light-emitting device isused in an indoor lighting device 8001. Since the light-emitting devicecan have a large area, it can be used for a lighting device having alarge area. In addition, with the use of a housing with a curvedsurface, a lighting device 8002 in which a light-emitting region has acurved surface can also be obtained. A light-emitting element includedin the light-emitting device described in this embodiment is in a thinfilm form, which allows the housing to be designed more freely. Thus,the lighting device can be elaborately designed in a variety of ways. Inaddition, a wall of the room may be provided with a lighting device8003.

Besides the above examples, when the light-emitting device is used aspart of furniture in a room, a lighting device that functions as thefurniture can be obtained.

In this manner, a variety of lighting devices to which thelight-emitting device is applied can be obtained.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 9

In this embodiment, touch panels including a light-emitting element ofone embodiment of the present invention or a light-emitting device ofone embodiment of the present invention will be described with referenceto FIGS. 13A and 13B, FIGS. 14A and 14B, FIGS. 15A and 15B, FIGS. 16Aand 16B, and FIG. 17.

FIGS. 13A and 13B are perspective views of a touch panel 2000. Note thatFIGS. 13A and 13B illustrate typical components of the touch panel 2000for simplicity.

The touch panel 2000 includes a display panel 2501 and a touch sensor2595 (see FIG. 13B). Furthermore, the touch panel 2000 includessubstrates 2510, 2570, and 2590.

The display panel 2501 includes a plurality of pixels over the substrate2510, and a plurality of wirings 2511 through which signals are suppliedto the pixels. The plurality of wirings 2511 are led to a peripheralportion of the substrate 2510, and part of the plurality of wirings 2511forms a terminal 2519. The terminal 2519 is electrically connected to anFPC 2509(1).

The substrate 2590 includes the touch sensor 2595 and a plurality ofwirings 2598 electrically connected to the touch sensor 2595. Theplurality of wirings 2598 are led to a peripheral portion of thesubstrate 2590, and part of the plurality of wirings 2598 forms aterminal 2599. The terminal 2599 is electrically connected to an FPC2509(2). Note that in FIG. 13B, electrodes, wirings, and the like of thetouch sensor 2595 provided on the back side of the substrate 2590 (theside facing the substrate 2510) are indicated by solid lines forclarity.

As the touch sensor 2595, a capacitive touch sensor can be used, forexample. Examples of the capacitive touch sensor are a surfacecapacitive touch sensor, a projected capacitive touch sensor, and thelike.

Examples of the projected capacitive touch sensor are a self-capacitivetouch sensor, a mutual capacitive touch sensor, and the like, whichdiffer mainly in the driving method. The use of a mutual capacitivetouch sensor is preferable because multiple points can be sensedsimultaneously.

First, an example of using a projected capacitive touch sensor isdescribed with reference to FIG. 13B. Note that in the case of aprojected capacitive touch sensor, a variety of sensors that can sensethe closeness or the contact of a sensing target such as a finger can beused.

The projected capacitive touch sensor 2595 includes electrodes 2591 and2592. The electrodes 2591 are electrically connected to any of theplurality of wirings 2598, and the electrodes 2592 are electricallyconnected to any of the other wirings 2598. The electrodes 2592 eachhave a shape of a plurality of quadrangles arranged in one directionwith one corner of a quadrangle connected to one corner of anotherquadrangle with a wiring 2594 in one direction, as illustrated in FIGS.13A and 13B. In the same manner, the electrodes 2591 each have a shapeof a plurality of quadrangles arranged with one corner of a quadrangleconnected to one corner of another quadrangle; however, the direction inwhich the electrodes 2591 are connected is a direction crossing thedirection in which the electrodes 2592 are connected. Note that thedirection in which the electrodes 2591 are connected and the directionin which the electrodes 2592 are connected are not necessarilyperpendicular to each other, and the electrodes 2591 may be arranged tointersect with the electrodes 2592 at an angle greater than 0° and lessthan 90°.

The intersecting area of the wiring 2594 and one of the electrodes 2592is preferably as small as possible. Such a structure allows a reductionin the area of a region where the electrodes are not provided, reducingunevenness in transmittance. As a result, unevenness in the luminance oflight from the touch sensor 2595 can be reduced.

Note that the shapes of the electrodes 2591 and the 2592 are not limitedto the above-described shapes and can be any of a variety of shapes. Forexample, the plurality of electrodes 2591 may be provided so that aspace between the electrodes 2591 are reduced as much as possible, andthe plurality of electrodes 2592 may be provided with an insulatinglayer sandwiched between the electrodes 2591 and 2592. In that case, itis preferable to provide, between two adjacent electrodes 2592, a dummyelectrode which is electrically insulated from these electrodes becausethe area of a region having a different transmittance can be reduced.

Next, the touch panel 2000 is described in detail with reference toFIGS. 14A and 14B. FIGS. 14A and 14B are cross-sectional views takenalong the dashed-dotted line X1-X2 in FIG. 13A.

The touch panel 2000 includes the touch sensor 2595 and the displaypanel 2501.

The touch sensor 2595 includes the electrodes 2591 and 2592 that areprovided in a staggered arrangement and in contact with the substrate2590, an insulating layer 2593 covering the electrodes 2591 and 2592,and the wiring 2594 that electrically connects the adjacent electrodes2591 to each other. Between the adjacent electrodes 2591, the electrode2592 is provided.

The electrodes 2591 and 2592 can be formed using a light-transmittingconductive material. As a light-transmitting conductive material, aconductive oxide such as indium oxide, indium tin oxide, indium zincoxide, zinc oxide, or zinc oxide to which gallium is added can be used.A graphene compound may be used as well. When a graphene compound isused, it can be formed, for example, by reducing a graphene oxide film.As a reducing method, a method with application of heat, a method withlaser irradiation, or the like can be employed.

For example, the electrodes 2591 and 2592 can be formed by depositing alight-transmitting conductive material on the substrate 2590 by asputtering method and then removing an unneeded portion by any ofvarious patterning techniques such as photolithography.

Examples of a material for the insulating layer 2593 are a resin such asacrylic or epoxy resin, a resin having a siloxane bond, and an inorganicinsulating material such as silicon oxide, silicon oxynitride, oraluminum oxide.

The adjacent electrodes 2591 are electrically connected to each otherwith the wiring 2594 formed in part of the insulating layer 2593. Notethat a material for the wiring 2594 preferably has higher conductivitythan materials for the electrodes 2591 and 2592 to reduce electricalresistance.

One wiring 2598 is electrically connected to any of the electrodes 2591and 2592. Part of the wiring 2598 serves as a terminal. For the wiring2598, a metal material such as aluminum, gold, platinum, silver, nickel,titanium, tungsten, chromium, molybdenum, iron, cobalt, copper, orpalladium or an alloy material containing any of these metal materialscan be used.

Through the terminal 2599, the wiring 2598 and the FPC 2509(2) areelectrically connected to each other. The terminal 2599 can be formedusing any of various kinds of anisotropic conductive films (ACF),anisotropic conductive pastes (ACP), and the like.

An adhesive layer 2597 is provided in contact with the wiring 2594. Thatis, the touch sensor 2595 is attached to the display panel 2501 so thatthey overlap with each other with the adhesive layer 2597 providedtherebetween. Note that the substrate 2570 as illustrated in FIG. 14Amay be provided over the surface of the display panel 2501 that is incontact with the adhesive layer 2597; however, the substrate 2570 is notalways needed.

The adhesive layer 2597 has a light-transmitting property. For example,a thermosetting resin or an ultraviolet curable resin can be used;specifically, a resin such as an acrylic-based resin, a urethane-basedresin, an epoxy-based resin, or a siloxane-based resin can be used.

The display panel 2501 in FIG. 14A includes, between the substrate 2510and the substrate 2570, a plurality of pixels arranged in a matrix and adriver circuit. Each pixel includes a light-emitting element and a pixelcircuit driving the light-emitting element.

In FIG. 14A, a pixel 2502R is shown as an example of the pixel of thedisplay panel 2501, and a scan line driver circuit 2503 g is shown as anexample of the driver circuit.

The pixel 2502R includes a light-emitting element 2550R and a transistor2502 t that can supply electric power to the light-emitting element2550R.

The transistor 2502 t is covered with an insulating layer 2521. Theinsulating layer 2521 covers unevenness caused by the transistor and thelike that have been already formed to provide a flat surface. Theinsulating layer 2521 may serve also as a layer for preventing diffusionof impurities. That is preferable because a reduction in the reliabilityof the transistor or the like due to diffusion of impurities can beprevented.

The light-emitting element 2550R is electrically connected to thetransistor 2502 t through a wiring. It is one electrode of thelight-emitting element 2550R that is directly connected to the wiring.An end portion of the one electrode of the light-emitting element 2550Ris covered with an insulator 2528.

The light-emitting element 2550R includes an EL layer between a pair ofelectrodes. A coloring layer 2567R is provided to overlap with thelight-emitting element 2550R, and part of light emitted from thelight-emitting element 2550R is transmitted through the coloring layer2567R and extracted in the direction indicated by an arrow in thedrawing. A light-blocking layer 2567BM is provided at an end portion ofthe coloring layer, and a sealing layer 2560 is provided between thelight-emitting element 2550R and the coloring layer 2567R.

Note that when the sealing layer 2560 is provided on the side from whichlight from the light-emitting element 2550R is extracted, the sealinglayer 2560 preferably has a light-transmitting property. The sealinglayer 2560 preferably has a higher refractive index than the air.

The scan line driver circuit 2503 g includes a transistor 2503 t and acapacitor 2503 c. Note that the driver circuit and the pixel circuitscan be formed in the same process over the same substrate. Thus, in amanner similar to that of the transistor 2502 t in the pixel circuit,the transistor 2503 t in the driver circuit (scan line driver circuit2503 g) is also covered with the insulating layer 2521.

The wirings 2511 through which a signal can be supplied to thetransistor 2503 t are provided. The terminal 2519 is provided in contactwith the wiring 2511. The terminal 2519 is electrically connected to theFPC 2509(1), and the FPC 2509(1) has a function of supplying signalssuch as an image signal and a synchronization signal. Note that aprinted wiring board (PWB) may be attached to the FPC 2509(1).

Although the case where the display panel 2501 illustrated in FIG. 14Aincludes a bottom-gate transistor is described, the structure of thetransistor is not limited thereto, and any of transistors with variousstructures can be used. In each of the transistors 2502 t and 2503 tillustrated in FIG. 14A, a semiconductor layer containing an oxidesemiconductor can be used for a channel region. Alternatively, asemiconductor layer containing amorphous silicon or a semiconductorlayer containing polycrystalline silicon that is obtained bycrystallization process such as laser annealing can be used for achannel region.

FIG. 14B illustrates the structure of the display panel 2501 thatincludes a top-gate transistor instead of the bottom-gate transistorillustrated in FIG. 14A. The kind of the semiconductor layer that can beused for the channel region does not depend on the structure of thetransistor.

In the touch panel 2000 illustrated in FIG. 14A, an anti-reflectionlayer 2567 p overlapping with at least the pixel is preferably providedon a surface of the touch panel on the side from which light from thepixel is extracted, as illustrated in FIG. 14A. As the anti-reflectionlayer 2567 p, a circular polarizing plate or the like can be used.

For the substrates 2510, 2570, and 2590 in FIG. 14A, for example, aflexible material having a vapor permeability of 1×10⁻⁵ g/(m²·day) orlower, preferably 1×10⁻⁶ g/(m²·day) or lower, can be favorably used.Alternatively, it is preferable to use the materials that make thesesubstrates have substantially the same coefficient of thermal expansion.For example, the coefficients of linear expansion of the materials are1×10⁻³/K or lower, preferably 5×10⁻⁵/K or lower and further preferably1×10⁻⁵/K or lower.

Next, a touch panel 2000 having a structure different from that of thetouch panel 2000 illustrated in FIGS. 14A and 14B is described withreference to FIGS. 15A and 15B. It can be used as a touch panel as wellas the touch panel 2000.

FIGS. 15A and 15B are cross-sectional views of the touch panel 2000. Inthe touch panel 2000′ illustrated in FIGS. 15A and 15B, the position ofthe touch sensor 2595 relative to the display panel 2501 is differentfrom that in the touch panel 2000 illustrated in FIGS. 14A and 14B. Onlydifferent structures are described below, and the above description ofthe touch panel 2000 can be referred to for the other similarstructures.

The coloring layer 2567R overlaps with the light-emitting element 2550R.Light from the light-emitting element 2550R illustrated in FIG. 15A isemitted to the side where the transistor 2502 t is provided. That is,(part of) light emitted from the light-emitting element 2550R passesthrough the coloring layer 2567R and is extracted in the directionindicated by an arrow in FIG. 15A. Note that the light-blocking layer2567BM is provided at an end portion of the coloring layer 2567R.

The touch sensor 2595 is provided on the transistor 2502 t side (the farside from the light-emitting element 2550R) of the display panel 2501(see FIG. 15A).

The adhesive layer 2597 is in contact with the substrate 2510 of thedisplay panel 2501 and attaches the display panel 2501 and the touchsensor 2595 to each other in the structure illustrated in FIG. 15A. Thesubstrate 2510 is not necessarily provided between the display panel2501 and the touch sensor 2595 that are attached to each other by theadhesive layer 2597.

As in the touch panel 2000, transistors with a variety of structures canbe used for the display panel 2501 in the touch panel 2000′. Although abottom-gate transistor is used in FIG. 15A, a top-gate transistor may beused as illustrated in FIG. 15B.

An example of a driving method of the touch panel is described withreference to FIGS. 16A and 16B.

FIG. 16A is a block diagram illustrating the structure of a mutualcapacitive touch sensor. FIG. 16A illustrates a pulse voltage outputcircuit 2601 and a current sensing circuit 2602. Note that in theexample of FIG. 16A, six wirings X1-X6 represent electrodes 2621 towhich a pulse voltage is supplied, and six wirings Y1-Y6 representelectrodes 2622 that sense a change in current. FIG. 16A alsoillustrates a capacitor 2603 which is formed in a region where theelectrodes 2621 and 2622 overlap with each other. Note that functionalreplacement between the electrodes 2621 and 2622 is possible.

The pulse voltage output circuit 2601 is a circuit for sequentiallyapplying a pulse voltage to the wirings X1 to X6. By application of apulse voltage to the wirings X1 to X6, an electric field is generatedbetween the electrodes 2621 and 2622 of the capacitor 2603. When theelectric field between the electrodes is shielded, for example, a changeoccurs in the capacitor 2603 (mutual capacitance). The approach orcontact of a sensing target can be sensed by utilizing this change.

The current sensing circuit 2602 is a circuit for sensing changes incurrent flowing through the wirings Y1 to Y6 that are caused by thechange in mutual capacitance in the capacitor 2603. No change in currentvalue is sensed in the wirings Y1 to Y6 when there is no approach orcontact of a sensing target, whereas a decrease in current value issensed when mutual capacitance is decreased owing to the approach orcontact of a sensing target. Note that an integrator circuit or the likeis used for sensing of current.

FIG. 16B is a timing chart showing input and output waveforms in themutual capacitive touch sensor illustrated in FIG. 16A. In FIG. 16B,sensing of a sensing target is performed in all the rows and columns inone frame period. FIG. 16B shows a period when a sensing target is notsensed (not touched) and a period when a sensing target is sensed(touched). Sensed current values of the wirings Y1 to Y6 are shown asthe waveforms of voltage values.

A pulse voltage is sequentially applied to the wirings X1 to X6, and thewaveforms of the wirings Y1 to Y6 change in accordance with the pulsevoltage. When there is no approach or contact of a sensing target, thewaveforms of the wirings Y1 to Y6 change uniformly in accordance withchanges in the voltages of the wirings X1 to X6. The current value isdecreased at the point of approach or contact of a sensing target andaccordingly the waveform of the voltage value changes. By sensing achange in mutual capacitance in this manner, the approach or contact ofa sensing target can be sensed.

Although FIG. 16A illustrates a passive touch sensor in which only thecapacitor 2603 is provided at the intersection of wirings as a touchsensor, an active touch sensor including a transistor and a capacitormay be used. FIG. 17 is a sensor circuit included in an active touchsensor.

The sensor circuit illustrated in FIG. 17 includes the capacitor 2603and transistors 2611, 2612, and 2613.

A signal G2 is input to a gate of the transistor 2613. A voltage VRES isapplied to one of a source and a drain of the transistor 2613, and oneelectrode of the capacitor 2603 and a gate of the transistor 2611 areelectrically connected to the other of the source and the drain of thetransistor 2613. One of a source and a drain of the transistor 2611 iselectrically connected to one of a source and a drain of the transistor2612, and a voltage VSS is applied to the other of the source and thedrain of the transistor 2611. A signal G1 is input to a gate of thetransistor 2612, and a wiring ML is electrically connected to the otherof the source and the drain of the transistor 2612. The voltage VSS isapplied to the other electrode of the capacitor 2603.

Next, the operation of the sensor circuit illustrated in FIG. 17 isdescribed. First, a potential for turning on the transistor 2613 issupplied as the signal G2, and a potential with respect to the voltageVRES is thus applied to a node n connected to the gate of the transistor2611. Then, a potential for turning off the transistor 2613 is appliedas the signal G2, whereby the potential of the node n is maintained.Then, mutual capacitance of the capacitor 2603 changes owing to theapproach or contact of a sensing target such as a finger; accordingly,the potential of the node n is changed from VRES.

In reading operation, a potential for turning on the transistor 2612 issupplied as the signal G1. A current flowing through the transistor2611, that is, a current flowing through the wiring ML is changed inaccordance with the potential of the node n. By sensing this current,the approach or contact of a sensing target can be sensed.

In each of the transistors 2611, 2612, and 2613, an oxide semiconductorlayer is preferably used as a semiconductor layer in which a channelregion is formed. In particular, such a transistor is preferably used asthe transistor 2613, so that the potential of the node n can be held fora long time and the frequency of operation of resupplying VRES to thenode n (refresh operation) can be reduced.

At least part of this embodiment can be implemented in combination withany of the embodiments described in this specification as appropriate.

Embodiment 10

In this embodiment, as a display device including a light-emittingelement, a display device which includes a reflective liquid crystalelement and a light-emitting element and is capable of performingdisplay both in a transmissive mode and a reflective mode is describedwith reference to FIGS. 36A, 36B1, and 36B2, FIG. 37, and FIG. 38.

The display device described in this embodiment can be driven withextremely low power consumption for display using the reflective mode ina bright place such as outdoors. Meanwhile, in a dark place such asindoors at night, image can be displayed at an optimal luminance withthe use of the transmissive mode. Thus, by combination of these modes,the display device can display an image with lower power consumption anda higher contrast compared to a conventional display panel.

As an example of the display device of this embodiment, description ismade on a display device in which a liquid crystal element provided witha reflective electrode and a light-emitting element are stacked and anopening of the reflective electrode is provided in a positionoverlapping with the light-emitting element. Visible light is reflectedby the reflective electrode in the reflective mode and light emittedfrom the light-emitting element is emitted through the opening of thereflective electrode in the transmissive mode. Note that transistorsused for driving these elements (the liquid crystal element and thelight-emitting element) are preferably formed on the same plane. It ispreferable that the liquid crystal element and the light-emittingelement be stacked through an insulating layer.

FIG. 36A is a block diagram illustrating a display device described inthis embodiment. A display device 600 includes a circuit (G) 601, acircuit (S) 602, and a display portion 603. In the display portion 603,a plurality of pixels 604 are arranged in an R direction and a Cdirection in a matrix. A plurality of wirings G1, wirings G2, wiringsANO, and wirings CSCOM are electrically connected to the circuit (G)601. These wirings are also electrically connected to the plurality ofpixels 604 arranged in the R direction. A plurality of wirings S1 andwirings S2 are electrically connected to the circuit (S) 602, and thesewirings are also electrically connected to the plurality of pixels 604arranged in the C direction.

Each of the plurality of pixels 604 includes a liquid crystal elementand a light-emitting element. The liquid crystal element and thelight-emitting element include portions overlapping with each other.

FIG. 36B1 shows the shape of a conductive film 605 serving as areflective electrode of the liquid crystal element included in the pixel604. Note that an opening 607 is provided in a position 606 which ispart of the conductive film 605 and which overlaps with thelight-emitting element. That is, light emitted from the light-emittingelement is emitted through the opening 607.

The pixels 604 in FIG. 36B1 are arranged such that adjacent pixels 604in the R direction exhibit different colors. Furthermore, the openings607 are provided so as not to be arranged in a line in the R direction.Such arrangement has an effect of suppressing crosstalk between thelight emitting elements of adjacent pixels 604.

The opening 607 can have a polygonal shape, a quadrangular shape, anelliptical shape, a circular shape, a cross shape, a stripe shape, or aslit-like shape, for example.

FIG. 36B2 illustrates another example of the arrangement of theconductive films 605.

The ratio of the opening 607 to the total area of the conductive film605 (excluding the opening 607) affects the display of the displaydevice. That is, a problem is caused in that as the area of the opening607 is larger, the display using the liquid crystal element becomesdarker; in contrast, as the area of the opening 607 is smaller, thedisplay using the light-emitting element becomes darker. Furthermore, inaddition to the problem of the ratio of the opening, a small area of theopening 607 itself also causes a problem in that extraction efficiencyof light emitted from the light-emitting element is decreased. The ratioof opening 607 to the total area of the conductive film 605 (other thanthe opening 607) is preferably 5% or more and 60% or less formaintaining display quality at the time of combination of the liquidcrystal element and the light-emitting element.

Next, an example of a circuit configuration of the pixel 604 isdescribed with reference to FIG. 37. FIG. 37 shows two adjacent pixels604.

The pixel 604 includes a transistor SW1, a capacitor C1, a liquidcrystal element 610, a transistor SW2, a transistor M, a capacitor C2, alight-emitting element 611, and the like. Note that these components areelectrically connected to any of the wiring G1, the wiring G2, thewiring ANO, the wiring CSCOM, the wiring S1, and the wiring S2 in thepixel 604. The liquid crystal element 610 and the light-emitting element611 are electrically connected to a wiring VCOM1 and a wiring VCOM2,respectively.

A gate of the transistor SW1 is connected to the wiring G1. One of asource and a drain of the transistor SW1 is connected to the wiring S1,and the other of the source and the drain is connected to one electrodeof the capacitor C1 and one electrode of the liquid crystal element 610.The other electrode of the capacitor C1 is electrically connected to thewiring CSCOM. The other electrode of the liquid crystal element 610 isconnected to the wiring VCOM1.

A gate of the transistor SW2 is connected to the wiring G2. One of asource and a drain of the transistor SW2 is connected to the wiring S2,and the other of the source and the drain is connected to one electrodeof the capacitor C2 and a gate of the transistor M. The other electrodeof the capacitor C2 is connected to one of a source and a drain of thetransistor M and the wiring ANO. The other of the source and the drainof the transistor M is connected to one electrode of the light-emittingelement 611. Furthermore, the other electrode of the light-emittingelement 611 is connected to the wiring VCOM2.

Note that the transistor M includes two gates between which asemiconductor is provided and which are electrically connected to eachother. With such a structure, the amount of current flowing through thetransistor M can be increased.

The on/off state of the transistor SW1 is controlled by a signal fromthe wiring G1. A predetermined potential is supplied from the wiringVCOM1. Furthermore, orientation of liquid crystals of the liquid crystalelement 610 can be controlled by a signal from the wiring S1. Apredetermined potential is supplied from the wiring CSCOM.

The on/off state of the transistor SW2 is controlled by a signal fromthe wiring G2. By the difference between the potentials applied from thewiring VCOM2 and the wiring ANO, the light-emitting element 611 can emitlight. Furthermore, the on/off state of the transistor M is controlledby a signal from the wiring S2.

Accordingly, in the structure of this embodiment, in the case of thereflective mode, the liquid crystal element 610 is controlled by thesignals supplied from the wiring G1 and the wiring S1 and opticalmodulation is utilized, whereby display can be performed. In the case ofthe transmissive mode, the light-emitting element 611 can emit lightwhen the signals are supplied from the wiring G2 and the wiring S2. Inthe case where both modes are performed at the same time, desireddriving can be performed based on the signals from the wiring G1, thewiring G2, the wiring S1, and the wiring S2.

Next, specific description will be given with reference to FIG. 38, aschematic cross-sectional view of the display device 600 described inthis embodiment.

The display device 600 includes a light-emitting element 623 and aliquid crystal element 624 between substrates 621 and 622. Note that thelight-emitting element 623 and the liquid crystal element 624 are formedwith an insulating layer 625 positioned therebetween. That is, thelight-emitting element 623 is positioned between the substrate 621 andthe insulating layer 625, and the liquid crystal element 624 ispositioned between the substrate 622 and the insulating layer 625.

A transistor 615, a transistor 616, a transistor 617, a coloring layer628, and the like are provided between the insulating layer 625 and thelight-emitting element 623.

A bonding layer 629 is provided between the substrate 621 and thelight-emitting element 623. The light-emitting element 623 includes aconductive layer 630 serving as one electrode, an EL layer 631, and aconductive layer 632 serving as the other electrode which are stacked inthis order over the insulating layer 625. In the light-emitting element623 that is a bottom emission light-emitting element, the conductivelayer 632 and the conductive layer 630 contain a material that reflectsvisible light and a material that transmits visible light, respectively.Light emitted from the light-emitting element 623 is transmitted throughthe coloring layer 628 and the insulating layer 625 and then transmittedthrough the liquid crystal element 624 via an opening 633, thereby beingemitted to the outside of the substrate 622.

In addition to the liquid crystal element 624, a coloring layer 634, alight-blocking layer 635, an insulating layer 646, a structure 636, andthe like are provided between the insulating layer 625 and the substrate622. The liquid crystal element 624 includes a conductive layer 637serving as one electrode, a liquid crystal 638, a conductive layer 639serving as the other electrode, alignment films 640 and 641, and thelike. Note that the liquid crystal element 624 is a reflective liquidcrystal element and the conductive layer 639 serves as a reflectiveelectrode; thus, the liquid crystal element 624 and the conductive layer639 are formed using a material with high reflectivity. Furthermore, theconductive layer 637 serves as a transparent electrode, and thus isformed using a material that transmits visible light. Alignment films640 and 641 may be provided on the conductive layers 637 and 638 and incontact with the liquid crystal layer 638. The insulating layer 646 isprovided so as to cover the coloring layer 634 and the light-blocking635 and serves as an overcoat layer. Note that the alignment films 640and 641 are not necessarily provided.

The opening 633 is provided in part of the conductive layer 639. Aconductive layer 643 is provided in contact with the conductive layer639 and has a light-transmitting property because of being formed usinga material transmitting visible light.

The structure 636 serves as a spacer that prevents the substrate 622from coming closer to the insulating layer 625 than required. Thestructure 636 is not necessarily provided.

One of a source and a drain of the transistor 615 is electricallyconnected to the conductive layer 630 in the light-emitting element 623.For example, the transistor 615 corresponds to the transistor M in FIG.37.

One of a source and a drain of the transistor 616 is electricallyconnected to the conductive layer 639 and the conductive layer 643 inthe liquid crystal element 624 through a terminal portion 618. That is,the terminal portion 618 electrically connects the conductive layersprovided on both surfaces of the insulating layer 625. The transistor616 corresponds to the switch SW1 in FIG. 37.

A terminal portion 619 is provided in a region where the substrates 621and 622 do not overlap with each other. Similarly to the terminalportion 618, the terminal portion 619 electrically connects theconductive layers provided on both surfaces of the insulating layer 625.The terminal portion 619 is electrically connected to a conductive layerobtained by processing the same conductive film as the conductive layer643. Thus, the terminal portion 619 and the FPC 644 can be electricallyconnected to each other through a connection layer 645.

A connection portion 647 is provided in part of a region where a bondinglayer 642 is provided. In the connection portion 647, the conductivelayer obtained by processing the same conductive film as the conductivelayer 643 and part of the conductive layer 637 are electricallyconnected with a connector 648. Accordingly, a signal or a potentialinput from the FPC 644 can be supplied to the conductive layer 637through the connection portion 647.

The structure 636 is provided between the conductive layer 637 and theconductive layer 643. The structure 636 maintains a cell gap of theliquid crystal element 624.

As the conductive layer 643, a metal oxide, a metal nitride, or an oxidesuch as an oxide semiconductor whose resistance is reduced is preferablyused. In the case of using an oxide semiconductor, a material in whichat least one of the concentrations of hydrogen, boron, phosphorus,nitrogen, and other impurities and the number of oxygen vacancies ismade to be higher than those in a semiconductor layer of a transistor isused for the conductive layer 643.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 11

In this embodiment, a light-emitting element is described. Thelight-emitting element described in this embodiment has a structuredifferent from that described in Embodiment 2. An element structure anda manufacturing method of the light-emitting element is described withreference to FIGS. 39A and 39B. For the portions similar to those inEmbodiments 2, the description of Embodiments 2 can be referred to anddescription is omitted.

The light-emitting element described in this embodiment has a structurein which an EL layer 3202 including a light-emitting layer 3213 issandwiched between a pair of electrodes (a cathode 3201 and an anode3203) formed over a substrate 3200. The EL layer 3202 can be formed bystacking a light-emitting layer, a hole-injection layer, ahole-transport layer, an electron-injection layer, an electron-transportlayer, and the like as in the EL layer described in Embodiment 2.

In this embodiment, as shown in FIG. 39A, description is made on thelight-emitting element having a structure in which the EL layer 3202including an electron-injection layer 3214, the light-emitting layer3213, a hole-transport layer 3215, and a hole-injection layer 3216 areformed over the cathode 3201 in this order over the substrate 3200 andthe anode 3203 is formed over the hole-injection layer 3216. Here,though an electron-transport layer is not provided, theelectron-injection layer 3214 can serve as the electron-transport layerwith a material having a high electron-transport property.

In the above-described light-emitting element, current flows due to apotential difference applied between the cathode 3201 and the anode3203, and holes and electrons recombine in the EL layer 3202, wherebylight is emitted. Then, this light emission is extracted to the outsidethrough one or both of the cathode 3201 and the anode 3203. Therefore,one or both of the cathode 3201 and the anode 3203 are electrodes havinglight-transmitting properties; light can be extracted through theelectrode having a light-transmitting property.

In the light-emitting element described in this embodiment, end portionsof the cathode 3201 are covered with insulators 3217 as shown in FIG.39. Note that the insulators 3217 are formed so as to fill a spacebetween adjacent cathodes 3201 (e.g., 3201 a and 3201 b) as shown inFIG. 39B.

As the insulator 3217, an inorganic compound or an organic compoundhaving an insulating property can be used. As the organic compound, aphotosensitive resin such as a resist material, e.g., an acrylic resin,a polyimide resin, a fluorine-based resin, or the like can be used. Asthe inorganic material, silicon oxide, silicon oxynitride, siliconnitride, or the like can be used, for example. Note that the insulator3217 preferably has a water-repellent surface. As its treatment method,plasma treatment, chemical treatment (using an alkaline solution or anorganic solvent), or the like can be employed.

In this embodiment, the electron-injection layer 3214 formed over thecathode 3201 is formed using a high molecular compound. It is preferableto use a high molecular compound which does not dissolve in thenonaqueous solvent and which has a high electron-transport property.Specifically, the electron-injection layer 3214 is formed using anappropriate combination of any of the materials (including not only ahigh molecular compound but also an alkali metal, an alkaline earthmetal, or a compound thereof) which can be used for theelectron-injection layer 115 and electron-transport layer 114 inEmbodiment 2. The materials are dissolved in a polar solvent, and thelayer is formed by a coating method.

Here, examples of the polar solvent include methanol, ethanol, propanol,isopropanol, butyl alcohol, ethylene glycol, and glycerin.

The light-emitting layer 3213 is formed over the electron-injectionlayer 3214. The light-emitting layer 3213 is formed by depositing (orapplying) ink in which any of the materials (a light-emitting substance)which can be used for the light-emitting layer 3213 in Embodiment 2 arecombined as appropriate and dissolved (dispersed) in a polar solvent, bya wet method (an ink-jet method or a printing method). Although theelectron-injection layer 3214 is used in common in light-emittingelements of different emission colors, a material corresponding to anemission color is selected for the light-emitting layer 3213. As thepolar solvent, an aromatic-based solvent such as toluene or xylene, or aheteroaromatic-based solvent such as pyridine can be used.Alternatively, a solvent such as hexane, 2-methylhexane cyclohexane, orchloroform can be used.

As shown in FIG. 39B, the ink for forming the light-emitting layer 3213is applied from a head portion 3300 of an apparatus for applying asolution (hereinafter referred to as solution application apparatus).Note that the head portion 3300 includes a plurality of sprayingportions 3301 a to 3301 c for spraying ink, and piezoelectric elements3302 a to 3302 c are provided for the spraying portions 3301 a to 3301c.

Furthermore, the spraying portions 3301 a to 3301 c are filled withrespective ink 3303 a to ink 3303 c containing emission substancesexhibiting different emission colors.

The ink 3303 a to ink 3303 c are sprayed from the respective sprayingportions 3301 a to 3301 c, whereby light-emitting layers 3213 a to 3213c emitting different colors are formed.

The hole-transport layer 3215 is formed over the light-emitting layer3213. The hole-transport layer 3215 can be formed by a combination ofany of the materials which can be used for the hole-transport layer 3215in Embodiment 2. The hole-transport layer 3215 can be formed by a vacuumevaporation method or a coating method. In the case of employing acoating method, the material which is dissolved in a solvent is appliedto the light-emitting layer 3213 and the insulator 3217. As a coatingmethod, an ink-jet method, a spin coating method, a printing method, orthe like can be used.

The hole-injection layer 3216 is formed over the hole-transport layer3215. The anode 3203 is formed over the hole-injection layer 3216. Theyare formed using an appropriate combination of the materials describedin Embodiment 2 by a vacuum evaporation method.

The light-emitting element can be formed through the above steps. Notethat in the case of using an organometallic complex in thelight-emitting layer, phosphorescence due to the organometallic complexis obtained. Thus, the light-emitting element can have higher efficiencythan a light-emitting element formed using only fluorescent compounds.

Note that the structure described in this embodiment can be used incombination with any of the structures described in the otherembodiments, as appropriate.

Embodiment 12

In this embodiment, an example of a lighting system to which alight-emitting element is applied, is described with reference to FIG.40.

As shown in FIG. 40, a lighting system 700 includes a plurality of lightemitting elements 701 and a control unit 702. The control unit 702 iselectrically connected to a power source 706. The control unit 702 iselectrically connected to each of the plurality of light-emittingelements 701 and controls lighting and extinction of each of thelight-emitting elements 701 and control the luminance at the time oflighting. That is, the lighting system of this embodiment is configuredto control emission of the light-emitting elements which are lightsources of the lighting in accordance with external information. Here,the external information includes brightness information, lighting timeinformation, and temperature information for example. To sense theexternal information, various parameters indicating such information canbe used as signals.

Any of the light-emitting elements described in the other embodimentscan be used for the light-emitting elements 701. An anode and a cathodeof the light-emitting element are electrically connected to the controlunit 702, and a potential of one of the electrodes or potentials of bothof the electrodes are controlled by a control signal from the controlunit 702, whereby lighting and extinction of the light-emitting elements701 and the luminance at the time of lighting are controlled.

The control unit 702 includes a central processing unit (CPU) 703, amemory 704, and a communication unit 705. Note that the communicationunit 705 is electrically connected to a sensor 707 and receives lightingtime information of the light-emitting elements 701 and externalbrightness information which are sensed by the sensor 707.

As the sensor 707, a photodiode, a light-receiving element such as a CdSphotoconductive element, a charge coupled device (CCD), a sensorincluding a CMOS sensor, or the like which can sense the above-describedinformation (e.g., lighting time information of the light-emittingelement 701 and external brightness information) as a data signal, canbe used.

In the memory 704, a program for controlling lighting time of thelight-emitting elements 701 is stored. The CPU 703 reads out from thememory 704 a signal (a lighting signal or an extinction signal) forlighting or extinguishing the plurality of light-emitting elements 701in accordance with lighting time information of the light-emittingelements 701 input from the communication unit 705, and executes drivingof the light-emitting elements 701. As an example of the program forcontrolling lighting time of the light-emitting elements 701, a programwith which consecutive lighting is completed in a certain period inorder to prevent unnecessary lighting for a long time, is given.

Note that a program for controlling the luminance of the light-emittingelements may be stored in the memory 704. In that case, the CPU 703reads out signals (luminance control signals) for controlling theluminance of the plurality of light-emitting elements 701 in accordancewith external brightness information input from the communication unit705, and executes driving of the light-emitting elements 701. An exampleof the program for controlling the luminance of the light-emittingelements includes a program with which luminance can be strengthened orweakened on the basis of whether the external brightness informationsensed by the sensor 707 reaches reference brightness set in advance.Furthermore, the signals for controlling the luminance of the pluralityof light-emitting elements 701 may be different for each of theplurality of the light-emitting elements 701 or may be the same.

Specifically, in the case where the CPU 703 executes driving of thelight-emitting element 701, the signal read out from the memory 704 isconverted by the CPU 703, and the potential of one electrode of thelight-emitting element is controlled.

Note that the plurality of the light-emitting elements 701 used in thelighting system of this embodiment may have the same structure ordifferent structures. Furthermore, emission colors of the plurality ofthe light-emitting elements 701 may be the same or different from eachother. The structure of the light-emitting element described in any ofthe other embodiments can be applied to the structure of thelight-emitting element in this embodiment.

Note that the structure described in this embodiment can be used incombination with any of the structures described in the otherembodiments, as appropriate.

EXAMPLE 1

In this example, light-emitting elements were fabricated andcharacteristic thereof were shown. Specifically, Light-emitting element1 in which only 1,5-bis[4-(9H-carbazole-9-yl)phenyl]anthracene(abbreviation: 1,5CzP2A) (Structural Formula: 100) was used in alight-emitting layer, Light-emitting element 2 in which 1,5CzP2A and1,6mMemFLPAPrn that was a dopant (a light-emitting substance) were usedin a light-emitting layer, Light-emitting element 3 in which only1,8-bis[4-(9H-carbazol-9-yl)phenyl]anthracene (abbreviation: 1,8CzP2A)(Structural Formula 110) was used in a light-emitting layer, andLight-emitting element 4 in which 1,8CzP2A and 1,6mMemFLPAPrn that was adopant (a light-emitting substance) were used in a light-emitting layer,were fabricated. Note that fabrication of Light-emitting elements 1 to 4is described with reference to FIG. 18. Chemical formulae of thematerials used in this example are shown below.

<<Fabrication of Light-Emitting Elements 1 to 4>>

First, indium tin oxide (ITO) containing silicon oxide was depositedover a glass substrate 900 by a sputtering method, whereby a firstelectrode 901 functioning as an anode was formed. The thickness of thefirst electrode 901 was set to 70 nm and the area of the electrode wasset to 2 mm ×2 mm.

Next, as pretreatment for fabricating the light-emitting element 1 overthe substrate 900, UV ozone treatment was performed for 370 secondsafter washing of a surface of the substrate with water and baking thatwas performed at 200° C. for 1 hour.

After that, the substrate 900 was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 1×10⁻⁴Pa, and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus. Then, the substrate900 was cooled down for approximately 30 minutes.

Next, the substrate 900 over which the first electrode 901 was formedwas fixed to a holder provided inside a vacuum evaporation apparatus sothat the surface over which the first electrode was formed faceddownward. In this example, a case will be described in which ahole-injection layer 911, a hole-transport layer 912, a light-emittinglayer 913, an electron-transport layer 914, and an electron-injectionlayer 915 which are included in an EL layer 902 are sequentially formedby a vacuum evaporation method.

The pressure in the vacuum evaporation apparatus was reduced to 1×10⁻⁴Pa Pa. Then, 3-[4-(9-Phenanthryl)-phenyl]-9-phenyl-9H-carbazole(abbreviation: PCPPn) and molybdenum oxide were deposited byco-evaporation with a mass ratio of PCPPn to molybdenum oxide being 4:2,thereby forming the first hole-injection layer 911 on the firstelectrode 901. A thickness thereof was set to be 10 nm. Note that aco-evaporation method is an evaporation method in which a plurality ofdifferent substances is concurrently vaporized from respective differentevaporation sources.

Next, PCPPn was deposited to a thickness of 20 nm by evaporation,thereby forming the hole-transport layer 912.

Next, the light-emitting layer 913 was formed on the hole-transportlayer 912.

In the case of Light-emitting element 1, the light-emitting layer 913was formed to a thickness of 25 nm by evaporation of 1,5CzP2A.

In the case of Light-emitting element 2, 1,5CzP2A and 1,6mMemFLPAPrnthat was a dopant (a light-emitting substance) were deposited byco-evaporation so that the mass ratio of 1,5CzP2A and 1,6mMemFLPAPrn was1:0.03, whereby the light-emitting layer 913 was formed to a thicknessof 25 nm.

In the case of Light-emitting element 3, the light-emitting layer 913was formed to a thickness of 25 nm by evaporation of 1,8CzP2A.

In the case of Light-emitting element 4, 1,8CzP2A and 1,6mMemFLPAPrnthat was a dopant (a light-emitting substance) were deposited byco-evaporation so that the mass ratio of 1,8CzP2A and 1,6mMemFLPAPrn was1:0.03, whereby the light-emitting layer 913 was formed to a thicknessof 25 nm.

Next, over the light-emitting layer 913,2,2′-(pyridine-2,6-diy)bis(4,6-diphenylpyrimidine) (abbreviation:2,6(P2Pm)2Py) was deposited to a thickness of 25 nm as theelectron-transport layer 914.

Furthermore, lithium fluoride was deposited by evaporation to athickness of 1 nm over the electron-transport layer 914, whereby theelectron-injection layer 915 was formed.

Lastly, aluminum was deposited to a thickness of 200 nm over theelectron-injection layer 915, whereby a second electrode 903 serving asa cathode was formed. Thus, Light-emitting elements 2 and 4 werefabricated. It is to be noted that an evaporation method using resistiveheating was employed for all the evaporation steps.

Table 1 shows element structures of Light-emitting elements 1 to 4obtained by the above steps.

TABLE 1 Hole- Hole- Light- Electron- Electron- First injection transportemitting transport injection Second electrode layer layer layer layerlayer electrode Light- ITO PCPPn:MoOx PCPPn 1,5CzPA 2,6(P2Pm) LiF Alemitting (70 nm) (4:2 (20 nm) 2Py (1 nm) (200 nm) element 1 10 nm) (25nm) Light- * emitting element 2 Light- 1,8CzPA emitting element 3 Light-** emitting element 4 * 1,5CzPA: 1,6mMemFLPAPrn (1:0.03 25 nm) **1,8CzPA: 1,6mMemFLPAPrn (1:0.03 25 nm)

The fabricated Light-emitting elements 1 to 4 were sealed in a glove boxunder a nitrogen atmosphere so as not to be exposed to the air (asealant was applied to surround the elements, and at the time ofsealing, UV treatment was performed and heat treatment was performed at80° C. for 1 hour).

<Delayed Fluorescence Measurement of Light-Emitting Element>

Delayed fluorescence measurement was performed on Light-emittingelements 1 to 4. A picosecond fluorescence lifetime measurement system(manufactured by Hamamatsu Photonics K.K.) was used for the measurement.To measure the lifetimes of fluorescence obtained from thelight-emitting layers of Light-emitting elements 1 to 4, thelight-emitting elements were made to emit light by applying a squarewave pulse voltage, and time-resolved measurements of light, which wasattenuated from the falling of the voltage, were performed using astreak camera. The pulse voltage was applied at a frequency of 10 Hz. Byintegrating data obtained by repeated measurements, data with a high S/Nratio was obtained. The measurement was performed at room temperature(in an atmosphere kept at 23° C.) under the conditions of a pulsevoltage of approximately 3 V, a pulse time width of 100 μsec, a negativebias voltage of −5 V, and a measurement time of 50 μsec.

The attenuation curves obtained by the measurement are shown in FIG. 19.In FIG. 19, the horizontal axis indicates the emission time (μs) elapsedafter the falling of the pulse voltage and the vertical axis indicatesthe relative emission intensity (arbitrary unit). Fitting of theattenuation curves shown in FIG. 19 was performed using Formula (5).

$\begin{matrix}{L = {\sum\limits_{n = 1}{A_{n\;}{\exp \left( {- \frac{t}{a_{n}}} \right)}}}} & (5)\end{matrix}$

In Formula (5), L and t represent normalized emission intensity andelapsed time, respectively.

As the results of the fitting of attenuation curves in FIG. 19, thefitting was able to be performed when n was 1 and 2 in Formula (5). Thefitting of the attenuation curves was performed and the proportion ofthe delayed fluorescence component in the total emission obtained fromeach of Light-emitting elements 1 to 4 was calculated by extrapolationof the fitting curves to t=0. As a result, the proportions of thedelayed fluorescence component in the total emission obtained fromLight-emitting element 1, Light-emitting element 2, Light-emittingelement 3, and Light-emitting element 4 were calculated to be 33%, 22%,10%, and 6%, respectively. In other words, 5% or more of the delayedfluorescence component was observed in each of the Light-emittingelements 1 to 4.

Light-emitting element 1 and Light-emitting element 2 included 1,5CzP2Ain the respective light-emitting layers, and Light-emitting element 3and Light-emitting element 4 included 1,8CzP2A in the respectivelight-emitting layers. That is, it was found that the proportion of thedelayed fluorescence component in each of Light-emitting element 1 andLight-emitting element 2 including 1,5CzP2A in its light-emitting layerwas higher than that of the delayed fluorescence component in each ofLight-emitting element 3 and Light-emitting element 4 including 1,8CzP2A in its light-emitting layer.

Note that a high proportion (specifically, 15% or more) of delayedfluorescence components in Light-emitting element 1 and Light-emittingelement 3 indicates that in 1,5CzP2A and 1,8CzP2A which are the organiccompounds, energy transfer from triplet excitons to singlet excitonsoccurs at a relatively high probability and that probability of TTA isincreased. Specifically, by using 1,5CzP2A having a particularly highproportion of delayed fluorescence components in the light-emittinglayer, the light-emitting element with high probability of TTA wasobtained. Note that this result agrees with the result in Embodiment 1in which 1,5CzP2A having a larger oscillator strength (f) than that in1,8CzP2A when comparison is performed using the oscillator strength (f),has a higher probability of TTA.

<<Operation Characteristics of Light-Emitting Elements 1 to 4>>

Operation characteristics of Light-emitting element 1 and Light-emittingelement 3 in which dopant (a light-emitting substance) was not includedin the respective light-emitting layers, and Light-emitting element 2and Light-emitting element 4 in which dopant (a light-emittingsubstance) was included in the respective light-emitting layers weremeasured. It is to be noted that the measurements were performed at roomtemperature (in an atmosphere kept at 25° C.).

FIG. 20 shows current density-luminance characteristics ofLight-emitting element 1 and Light-emitting element 3 in which dopant (alight-emitting substance) was not included in the respectivelight-emitting layers. FIG. 21 shows voltage-luminance characteristicsthereof. FIG. 22 shows luminance-current efficiency characteristicsthereof. FIG. 23 shows voltage-current characteristics thereof.

Table 2 shows initial values of main characteristics of Light-emittingelement 1 and Light-emitting element 3 at a luminance of about 1000cd/m².

TABLE 2 Current Current Power Voltage Current density ChromaticityLuminance density efficiency External quantum (V) (mA) (mA/cm²) (x, y)(cd/m²) (cd/A) (lm/W) efficiency (%) Light- 4.4 1.1 27 (0.15, 0.13) 7602.8 2.0 2.6 emitting elemet 1 Light- 5.4 1.5 37 (0.15, 0.11) 1000 2.71.6 2.8 emitting element 3

FIG. 24 shows current density-luminance characteristics ofLight-emitting elements 2 and 4, FIG. 25 shows voltage-luminancecharacteristics thereof, FIG. 26 shows luminance-current efficiencycharacteristics thereof, and FIG. 27 shows voltage-currentcharacteristics thereof.

Table 4 shows initial values of main characteristics of Light-emittingelements 2 and 3 at a luminance of approximately 1000 cd/m².

TABLE 3 Current Current Power External Voltage Current densityChromaticity Luminance density efficiency quantum (V) (mA) (mA/cm²) (x,y) (cd/m²) (cd/A) (lm/W) efficiency (%) Light- 4.0 0.41 10 (0.14, 0.15)800 7.8 6.1 7.0 emitting elemet 2 Light- 5.0 0.60 15 (0.14, 0.16) 11007.6 4.8 6.5 emitting element 4

When the external quantum efficiencies of Light-emitting elements 2 and4 containing dopant (here, 1,6mMemFLPAPrn) are compared with each other,Light-emitting element 2 in which 1,5CzP2A was used as a host materialused in the light-emitting layer of the light-emitting element had ahigher external quantum efficiency than that of Light-emitting element4. This indicated that the use of the material (1,5CzP2A) having alarger oscillator strength (f) was one factor allowing easy occurrenceof TTA.

FIG. 28 shows emission spectra of Light-emitting elements 1 and 3 towhich current was applied at a current density of 25 mA/cm². FIG. 29shows an emission spectrum of Light-emitting elements 2 and 4, to whichcurrent was applied at a current density of 25 mA/cm².

EXAMPLE 2 Synthesis Example 1

In this example, a synthesis example of an organic compound,1,5-bis[4-(9H-carbazole-9-yl)phenyl]anthracene (abbreviation: 1,5CzP2A)(Structural Formula:100) is described. Note that chemical formula of1,5CzP2A is shown below.

<Synthesis of 1,5CzP2A>

A mixture of 1.2 g (3.6 mmol) of dibromoanthracene, 2.3 g (7.9 mmol) of4-(9H-carbazol-9-yl)phenylboronic acid, 2.2 g (16 mmol) of potassiumcarbonate, 30 mL of toluene, and 10 mL of ethanol, 8 mL of water, and 83mg (71 μmol of tetrakis(triphenylphosphine)palladium(0) was stirredunder a nitrogen stream at 90° C. for 14 hours.

After the stirring, the mixture was filtered, the obtained solid waswashed with water and ethanol and then collected. This solid waspurified by silica gel column chromatography (developing solvent:toluene). The purified solid was recrystallized, so that 2.0 g of a paleyellow solid was obtained in a yield of 86%.

The 2.0 g of obtained solid was purified by a train sublimation methodunder a pressure of 2.7 Pa in an argon stream at 343° C. After thepurification, 1.8 g of a pale yellow solid was obtained at a collectionrate of 90%. Synthesis scheme (a) of the above synthesis method is shownbelow.

The following shows analysis results by nuclear magnetic resonance(¹H-NMR) spectroscopy of the pale yellow solid obtained by theabove-described synthesis method. A ¹H-NMR chart is shown in FIGS. 30Aand 30B. The ¹H NMR charts revealed that 1,5CzP2A, the organic compoundof one embodiment of the present invention represented by StructuralFormula (100), was obtained in Synthesis Example 1.

¹H-NMR (CDCl₃, 300 MHz) : δ=7.36 (t, J₁=7.8 Hz, 4H), 7.51 (t, J₁=8.4 Hz,4H),7.57 (s, 2H), 7.58 (dd, J₁=6.9 Hz, J₂=11.7 Hz, 2H), 7.65 (d, J₁=7.8Hz, 4H), 7.80(d, J₁=8.4 Hz, 4H), 7.88 (d, J₁=8.7 Hz, 4H), 8.07 (dd,J₁=2.4 Hz, J₂=6.6 Hz, 2H), 8.22 (dd, J₁=7.5 H, 4H), 8.72 (s=2H).

Next, ultraviolet-visible absorption spectra (hereinafter simplyreferred to as “absorption spectra”) and emission spectra of 1,5CzP2A ina toluene solution and 1,5CzP2A in a solid thin film were measured. Thesolid thin film was formed over a quartz substrate by a vacuumevaporation method. The absorption spectra were measured using anultraviolet-visible light spectrophotometer (V-550 type manufactured byJASCO Corporation). The emission spectra were measured using afluorescence spectrophotometer (FS920 manufactured by HamamatsuPhotonics K.K.). FIG. 31A shows the obtained absorption and emissionspectra of 1,5CzP2A in the toluene solution. The horizontal axisrepresents wavelength, and the vertical axis represents absorptionintensity. FIG. 31B shows the obtained absorption and emission spectraof 1,5CzP2A in the solid thin film. The horizontal axis representswavelength, and the vertical axis represents absorption intensity. Theabsorption spectrum shown in FIG. 31A is a result obtained bysubtraction of an absorption spectrum of toluene only put in a quartzcell from the measured absorption spectrum of the toluene solution in aquartz cell. The absorption spectrum of the thin film in FIG. 31B wasobtained by subtraction of an absorption spectrum of the quartzsubstrate from an absorption spectrum of the quartz on which 1,5CzP2Awas deposited.

FIG. 31A shows that 1,5CzP2A in the toluene solution has absorptionpeaks at around 287 nm, 293 nm, 327 nm, 341 nm, 359 nm, 378 nm, and 397nm, and emission wavelength peaks at around 425 nm and 448 nm. (Theexcitation wavelength: 379 nm). FIG. 31B shows that 1,5CzP2A in thesolid thin film has absorption peaks at around 265 nm, 286 nm, 296 nm,314 nm, 331 nm, 345 nm, 369 nm, 387 nm, and 404 nm, and an emissionwavelength peak at around 462 nm. (The excitation wavelength: 345 nm).

Next, LC/MS analysis was performed. FIG. 32 shows the measurementresults.

In the LC/MS analysis, liquid chromatography (LC) separation was carriedout with ACQUITY UPLC® (manufactured by Waters Corporation) and massspectrometry (MS) analysis was carried out with Xevo G2 Tof MS(manufactured by Waters Corporation).

For the LC separation, Acquity UPLC BEH C8 (2.1×100 mm, 1.7 μm) was usedas a column, and a mixed solution of acetonitrile and a 0.1% formic acidaqueous solution was used for a mobile phase.

In the MS analysis, ionization was carried out by an electrosprayionization (ESI) method, and the analysis was performed in a positivemode. A component that underwent the ionization was collided with anargon gas in a collision cell to dissociate into product ions. Energy(collision energy) for the collision with argon was 50 eV. The massrange for the measurement was mz=100 to 1200.

The result shows that precursor ions of 1,5CzP2A were detected aroundm/z=661, and product ions of 1,5CzP2A were detected around m/z=495 andm/z=707. These results are characteristically derived from 1,5CzP2A andthus can be regarded as important data in identification of 1,5CzP2Acontained in the mixture.

Note that the product ion around m/z=495 is presumed to be a hydrogenion adduct of a radical expressed as C₃₈H₂₅N^(·−) in the state where onecarbazole is dissociated, and the product ion around m/z=707 is presumedto be acetonitrile and a hydrogen ion adduct. These indicate that aterminal of 1,5CzP2A has a carbazole skeleton and that acetonitrile iseasily added. Note that acetonitrile was used for sample adjustment atthe time of the analysis and for the mobile phase. Note that there is apossibility that the above m/z values±1 are detected as protonation ordeprotonation products of the product ions.

Note that the synthesis method of 1,5CzP2A which is the organic compoundof one embodiment of the present invention described in this example isa preferable example; however, the present invention is not limited tothis example and another synthesis method can be employed.

The above-described organic compound, 1,5CzP2A can be used not only as ahost material of the light-emitting layer but also as a light-emittingsubstance.

EXAMPLE 3 Synthesis Example 2

In this example, a synthesis method of an organic compound,1,8-bis[4-(9H-carbazole-9-yl)phenyl]anthracene (abbreviation: 1,8CzP2A)is described. Note that chemical formula of 1,8CzP2A is shown below.

<Synthesis of 1,8CzP2A>

In a 200 mL three-neck flask, a mixture of 1.2 g (3.7 mmol) ofdibromoanthracene, 2.3 g (8.1 mmol) of 4-(9H-carbazol-9-yl)phenylboronicacid, 2.2 g (16 mmol) of potassium carbonate, 30 mL of toluene, and 10mL of ethanol, 8 mL of water, and 85 mg (74 ₁μmol) oftetrakis(triphenylphosphine)palladium(0) was stirred under a nitrogenstream at 90° C. for 14 hours.

After the stifling, the mixture was filtered, the obtained solid waswashed with water and ethanol and then collected. This solid waspurified by silica gel column chromatography (developing solvent:toluene). The purified solid was recrystallized by toluene, so that 2.3g of a pale yellow solid was obtained in a yield of 93%. 2.0 g of theobtained solid was purified by a train sublimation method under apressure of 2.7 Pa in an argon stream at 295° C. After the purification,1.9 g of a pale yellow solid was obtained at a collection rate of 83%.Synthesis scheme (b) of the above synthesis method is shown below.

The following shows analysis results by nuclear magnetic resonance(¹H-NMR) spectroscopy of the pale yellow solid obtained by theabove-described synthesis method. A ¹H-NMR chart is shown in FIGS. 33Aand 33B. The ¹H-NMR charts revealed that 1,8CzP2A, the organic compoundof one embodiment of the present invention represented by StructuralFormula (110), was obtained in Synthesis Example 2.

¹H NMR (CDCl₃, 300 MHz) : δ=6.88 (t, J₁=7.2 Hz, 4H), 7.08 (t, J₁=7.8 Hz,4H), 7.31(d, J₁=8.1 Hz, 4H), 7.55 (dd, J₁=1.5 Hz, J₂=6.9 Hz, 2H),7.60-7.68 (m, 6H), 7.80 (d, J₁=8.1 Hz, 4H), 8.03 (d, J₁=7.8 Hz, 4H),8.14 (d, J₁=7.8 Hz, 2H), 8.66 (s, 1H), 8.95 (s, 1H).

Next, ultraviolet-visible absorption spectra (hereinafter simplyreferred to as “absorption spectra”) and emission spectra of 1,8CzP2A ina toluene solution and 1,8CzP2A in a solid thin film were measured. Thetoluene solution and the solid thin film were each measured in a mannersimilar to that in Example 2. FIG. 34A shows the obtained absorption andemission spectra of 1,8CzP2A in the toluene solution. The horizontalaxis represents wavelength, and the vertical axis represents absorptionintensity. FIG. 34B shows the obtained absorption and emission spectraof the solid thin film. The horizontal axis represents wavelength, andthe vertical axis represents absorption intensity.

FIG. 34A shows that 1,8CzP2A in the toluene solution has absorptionpeaks at around 287 nm, 294 nm, 328 nm, 341 nm, 361 nm, 380 nm, and 399nm, and emission wavelength peaks at around 423 nm and 445 nm. (Theexcitation wavelength: 381 nm). FIG. 34B shows that 1,8CzP2A in thesolid thin film has absorption peaks at around 265 nm, 286 nm, 296 nm,315 nm, 331 nm, 344 nm, 370 nm, 388 nm, and 404 nm, and an emissionwavelength peak at around 468 nm. (The excitation wavelength: 345 nm).

Next, LC/MS analysis was performed. The measurement method was similarto that used in Example 2. FIG. 35 shows the measurement results.

The result shows that precursor ions of 1,8CzP2A were detected aroundm/z=661, and product ions of 1,8CzP2A were detected around m/z=243,m/z=329, and m/z=495. The results are characteristically derived from1,8CzP2A and thus can be regarded as important data in identification of1,8CzP2A contained in a mixture.

Note that the product ion around m/z=243 is presumed to be a protonadduct of a radical of phenylcarbazole expressed as C₁₈H₁₃N^(·+); theproduct ion around m/z=329 is presumed to be a hydrogen ion adduct of abiradical expressed as C₂₆H₁₇₂ ^(·+) in the state where two carbazolesare dissociated; and the product ion around m/z=495 is presumed to be ahydrogen ion adduct of a radical expressed as C₃₈H₂₅ ^(·+) in the statewhere one carbazole is dissociated. These indicate that a terminal of1,8CzP2A includes two carbazole skeletons and a phenylcarbazoleskeleton. Note that there is a possibility that the above m/z values±1are detected as protonation or deprotonation products of the productions.

Note that the synthesis method of 1,8CzP2A which is the organic compoundof one embodiment of the present invention described in this example isa preferable example; however, the present invention is not limited tothis example and another synthesis method can be employed.

The above-described organic compound, 1,8CzP2A can be used as not onlyas a host material of the light-emitting layer but also as alight-emitting substance.

This application is based on Japanese Patent Application serial no.2015-146604 filed with Japan Patent Office on Jul. 24, 2015 and JapanesePatent Application serial no. 2016-006140 filed with Japan Patent Officeon Jan. 15, 2016, the entire contents of which are hereby incorporatedby reference.

What is claimed is:
 1. A light-emitting element comprising: an EL layerbetween an anode and a cathode, wherein the EL layer includes alight-emitting layer, wherein the light-emitting layer includes a firstorganic compound, and wherein a difference between a T₁ level of thefirst organic compound and one or more of T_(n) levels of the firstorganic compound is less than the sum of the T₁ level and 0.6 eV.
 2. Thelight-emitting element according to claim 1, wherein an energydifference between the one or more of T_(n) levels of the first organiccompound and any one of an S₁ level and S_(n) levels of the firstorganic compound is 1 eV or less.
 3. The light-emitting elementaccording to claim 1, wherein an oscillator strength for excitation fromthe T₁ level of the first organic compound to the one or more of T_(n)levels of the first organic compound is 0.0015 or more.
 4. Thelight-emitting element according to claim 1, wherein the first organiccompound includes a tetracene skeleton or an anthracene skeleton.
 5. Thelight-emitting element according to claim 1, wherein the first organiccompound has Structural Formula (100) or Structural Formula (110).


6. The light-emitting element according to claim 1, wherein lightemission from the light-emitting element includes a delayed fluorescencecomponent, and wherein an emission intensity of the delayed fluorescencecomponent is 5% or more to that of a total amount of the light emission.7. The light-emitting element according to claim 6, wherein thelight-emitting layer further includes a second organic compound, whereinan S₁ level of the first organic compound is higher an S₁ level of thesecond organic compound, and wherein the light emission from thelight-emitting element is derived from the second organic compound.
 158. The light-emitting element according to claim 7, wherein the secondorganic compound includes a pyrene skeleton.
 9. The light-emittingelement according to claim 7, wherein a T₁ level of the second organiccompound is higher than the T₁ level of the first organic compound. 10.The light-emitting element according to claim 7, wherein the EL layerfurther includes a hole-transport layer including a third organiccompound, wherein the hole-transport layer is located between the anodeand the light-emitting layer and in contact with the light-emittinglayer, and wherein a T₁ level of the third organic compound included inthe hole-transport layer is higher than the T₁ level of the firstorganic compound.
 11. The light-emitting element according to claim 10,wherein the EL layer further includes an electron-transport layerincluding a fourth organic compound, wherein the electron-transportlayer is located between the cathode and the light-emitting layer and incontact with the light-emitting layer, and wherein a T₁ level of thefourth organic compound included in the electron-transport layer ishigher than the T₁ level of the first organic compound.
 12. Alight-emitting device comprising: the light-emitting element accordingto claim 1, and one of a transistor and a substrate.
 13. An electronicdevice comprising: the light-emitting device according to claim 12, andany one of microphone, a camera, an operation button, an externalconnection portion, and a speaker.
 14. An electronic device comprising:the light-emitting device according to claim 12, and one of a housingand a touch sensor.
 15. A lighting device comprising: the light-emittingdevice according to claim 12, and any one of a housing, a cover, and asupport.
 16. A lighting system comprising: a sensor; a control unitelectrically connected to the sensor; and a light-emitting elementelectrically connected to the control unit, wherein information acquiredby the sensor is input to the control unit, wherein the control unit isconfigured to drive the light-emitting element based on the information,wherein the light-emitting element includes an EL layer between an anodeand a cathode, wherein the EL layer includes a light-emitting layer,wherein the light-emitting layer includes a first organic compound, andwherein a difference between a T₁ level of the first organic compoundand one or more of T_(n) levels of the first organic compound is lessthan the sum of the T₁ level and 0.6 eV.
 17. The lighting systemaccording to claim 16, wherein an energy difference between the one ormore of T_(n) levels of the first organic compound and any one of an S₁level and S_(n) levels of the first organic compound is 1 eV or less.18. A lighting system comprising: a sensor; a control unit; and alight-emitting element, wherein the control unit includes acommunication unit, a CPU, and a memory, wherein the memory includes aprogram for driving the light-emitting element based on exteriorinformation, wherein the communication unit is configured to send theexterior information acquired by the sensor to the CPU, wherein the CPUis configured to drive the light-emitting element by reading out theprogram from the memory and executing the program, wherein thelight-emitting element includes an EL layer between an anode and acathode, wherein the EL layer includes a light-emitting layer, whereinthe light-emitting layer includes a first organic compound, and whereina difference between a T₁ level of the first organic compound and one ormore of T_(n) levels of the first organic compound is less than the sumof the T₁ level and 0.6 eV.
 19. The lighting system according to claim18, wherein an energy difference between the one or more of T_(n) levelsof the first organic compound and any one of an S₁ level and S_(n)levels of the first organic compound is 1 eV or less.